Bump minimum tools version to macOS 11.0 (#633)

This fixes a warning with Xcode 27:

```
/Applications/Xcode-27.0.0-beta5.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/__configuration/availability.h:204:4: warning:
      "The selected platform is no longer supported by libc++." [-W#warnings]
  204 | #  warning "The selected platform is no longer supported by libc++."
      |    ^
```

crosstool/universal_exec_tool.bzl

diff --git a/crosstool/universal_exec_tool.bzl b/crosstool/universal_exec_tool.bzl
index df60c37..306eb06 100644
--- a/crosstool/universal_exec_tool.bzl
+++ b/crosstool/universal_exec_tool.bzl
@@ -33,7 +33,7 @@
   xcrun \
     --sdk macosx \
     clang \
-    -mmacosx-version-min=10.15 \
+    -mmacosx-version-min=11.0 \
     -std=c++17 \
     -lc++ \
     -arch arm64 \

xcode/xcode_configure.bzl

diff --git a/xcode/xcode_configure.bzl b/xcode/xcode_configure.bzl
index 669dc56..9618a43 100644
--- a/xcode/xcode_configure.bzl
+++ b/xcode/xcode_configure.bzl
@@ -137,7 +137,7 @@
         "--sdk",
         "macosx",
         "clang",
-        "-mmacosx-version-min=10.13",
+        "-mmacosx-version-min=11.0",
         "-fobjc-arc",
         "-framework",
         "CoreServices",