Replace gcc-9 with gcc-11 for macOS testing.

At some point, macos-latest stopped supporting gcc-9, and moved to
gcc-11 and gcc-12.
diff --git a/.github/workflows/cmake.yml b/.github/workflows/cmake.yml
index 64aa12c..d15fda8 100644
--- a/.github/workflows/cmake.yml
+++ b/.github/workflows/cmake.yml
@@ -54,7 +54,7 @@
 
           - name: macOS GCC
             os: macos-latest
-            compiler: gcc-9
+            compiler: gcc-11
 
     steps:
     - name: Checkout repository
diff --git a/.github/workflows/configure.yml b/.github/workflows/configure.yml
index 712c723..e783921 100644
--- a/.github/workflows/configure.yml
+++ b/.github/workflows/configure.yml
@@ -85,7 +85,7 @@
 
           - name: macOS GCC
             os: macos-latest
-            compiler: gcc-9
+            compiler: gcc-11
             configure-args: --warn
 
           - name: macOS Clang