Update minimum rules_go version to 0.13.0 for gazelle 0.13.0 (#284)

This should have been done before the 0.13.0 release. It caused some
confusion in #272. Note that a warning is only printed for the minimum
version in "fix" mode.
diff --git a/README.rst b/README.rst
index ce2a3b5..a47385a 100644
--- a/README.rst
+++ b/README.rst
@@ -139,7 +139,7 @@
 +---------------------+------------------------------+------------------------------+
 | 0.12.0              | 0.11.0                       | n/a                          |
 +---------------------+------------------------------+------------------------------+
-| 0.13.0              | 0.11.0                       | n/a                          |
+| 0.13.0              | 0.13.0                       | n/a                          |
 +---------------------+------------------------------+------------------------------+
 
 Usage
diff --git a/cmd/gazelle/version.go b/cmd/gazelle/version.go
index 5f895a9..553ae8a 100644
--- a/cmd/gazelle/version.go
+++ b/cmd/gazelle/version.go
@@ -26,7 +26,7 @@
 	"github.com/bazelbuild/bazel-gazelle/internal/version"
 )
 
-var minimumRulesGoVersion = version.Version{0, 11, 0}
+var minimumRulesGoVersion = version.Version{0, 13, 0}
 
 // checkRulesGoVersion checks whether a compatible version of rules_go is
 // being used in the workspace. A message will be logged if an incompatible