Bump min CMake version to 3.15

Now we can use TARGET_FILE_BASE_NAME
diff --git a/32blit-pico/executable.cmake b/32blit-pico/executable.cmake
index 462836b..4ef3540 100644
--- a/32blit-pico/executable.cmake
+++ b/32blit-pico/executable.cmake
@@ -118,13 +118,7 @@
         pico_add_bin_output(${NAME})
         pico_add_dis_output(${NAME})
 
-        # Ideally we want the .blit filename to match the .elf, but TARGET_FILE_BASE_NAME isn't always available
-        # (This only affects the firmware updater as it's the only thing setting a custom OUTPUT_NAME)
-        if(${CMAKE_VERSION} VERSION_LESS "3.15.0")
-            set(BLIT_FILENAME ${NAME}.blit)
-        else()
-            set(BLIT_FILENAME $<TARGET_FILE_BASE_NAME:${NAME}>.blit)
-        endif()
+        set(BLIT_FILENAME $<TARGET_FILE_BASE_NAME:${NAME}>.blit)
 
         # no relocs, just copy it
         set(BIN_NAME "$<IF:$<BOOL:$<TARGET_PROPERTY:${NAME},OUTPUT_NAME>>,$<TARGET_PROPERTY:${NAME},OUTPUT_NAME>,$<TARGET_PROPERTY:${NAME},NAME>>.bin")
@@ -179,11 +173,7 @@
         target_compile_definitions(${TARGET} PRIVATE PICO_NO_BI_PROGRAM_NAME=1)
     else()
         # real .blit metadata
-        if(${CMAKE_VERSION} VERSION_LESS "3.15.0")
-            set(BLIT_FILENAME ${TARGET}.blit)
-        else()
-            set(BLIT_FILENAME $<TARGET_FILE_BASE_NAME:${TARGET}>.blit)
-        endif()
+        set(BLIT_FILENAME $<TARGET_FILE_BASE_NAME:${TARGET}>.blit)
 
         add_custom_command(
             TARGET ${TARGET} POST_BUILD
diff --git a/32blit-stm32/executable.cmake b/32blit-stm32/executable.cmake
index d354838..3eaed65 100644
--- a/32blit-stm32/executable.cmake
+++ b/32blit-stm32/executable.cmake
@@ -32,13 +32,7 @@
 
 	add_executable(${NAME} ${USER_STARTUP} ${SOURCES})
 
-	# Ideally we want the .blit filename to match the .elf, but TARGET_FILE_BASE_NAME isn't always available
-	# (This only affects the firmware updater as it's the only thing setting a custom OUTPUT_NAME)
-	if(${CMAKE_VERSION} VERSION_LESS "3.15.0")
-		set(BLIT_FILENAME ${NAME}.blit)
-	else()
-		set(BLIT_FILENAME $<TARGET_FILE_BASE_NAME:${NAME}>.blit)
-	endif()
+	set(BLIT_FILENAME $<TARGET_FILE_BASE_NAME:${NAME}>.blit)
 
 	install(FILES ${CMAKE_CURRENT_BINARY_DIR}/${BLIT_FILENAME}
 		DESTINATION bin
@@ -80,11 +74,7 @@
 
 	include(${CMAKE_CURRENT_BINARY_DIR}/metadata.cmake)
 
-	if(${CMAKE_VERSION} VERSION_LESS "3.15.0")
-		set(BLIT_FILENAME ${TARGET}.blit)
-	else()
-		set(BLIT_FILENAME $<TARGET_FILE_BASE_NAME:${TARGET}>.blit)
-	endif()
+	set(BLIT_FILENAME $<TARGET_FILE_BASE_NAME:${TARGET}>.blit)
 
 	add_custom_command(
 		TARGET ${TARGET} POST_BUILD
diff --git a/32blit-stm32/firmware/CMakeLists.txt b/32blit-stm32/firmware/CMakeLists.txt
index a95fc9c..4472a6b 100644
--- a/32blit-stm32/firmware/CMakeLists.txt
+++ b/32blit-stm32/firmware/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.12...3.31)
+cmake_minimum_required(VERSION 3.15...3.31)
 project (firmware)
 find_package (32BLIT CONFIG REQUIRED PATHS ..)
 
diff --git a/CMakeLists.txt b/CMakeLists.txt
index d11a354..2456074 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.12...3.31)
+cmake_minimum_required(VERSION 3.15...3.31)
 
 # has to be before project
 include(32blit-pico/sdk_import.cmake)
diff --git a/launcher/CMakeLists.txt b/launcher/CMakeLists.txt
index 079d55a..d229731 100644
--- a/launcher/CMakeLists.txt
+++ b/launcher/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.12...3.31)
+cmake_minimum_required(VERSION 3.15...3.31)
 project (launcher)
 find_package (32BLIT CONFIG REQUIRED PATHS ..)
 
diff --git a/utilities/CMakeLists.txt b/utilities/CMakeLists.txt
index 23a29aa..0dbc018 100644
--- a/utilities/CMakeLists.txt
+++ b/utilities/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.12...3.31)
+cmake_minimum_required(VERSION 3.15...3.31)
 project (examples)
 find_package (32BLIT CONFIG REQUIRED PATHS ..)
 
diff --git a/utilities/hardware-test/CMakeLists.txt b/utilities/hardware-test/CMakeLists.txt
index eeda9b0..f42901c 100644
--- a/utilities/hardware-test/CMakeLists.txt
+++ b/utilities/hardware-test/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.12...3.31)
+cmake_minimum_required(VERSION 3.15...3.31)
 project (hardware-test)
 find_package (32BLIT CONFIG REQUIRED PATHS ../..)
 
diff --git a/utilities/picosystem-hardware-test/CMakeLists.txt b/utilities/picosystem-hardware-test/CMakeLists.txt
index eb264f8..bf1a13c 100644
--- a/utilities/picosystem-hardware-test/CMakeLists.txt
+++ b/utilities/picosystem-hardware-test/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.12...3.31)
+cmake_minimum_required(VERSION 3.15...3.31)
 project (picosystem-hardware-test)
 find_package (32BLIT CONFIG REQUIRED PATHS ../..)