Update CI to macos-13. (#1207)

Drop macos-12, which is now deprecated, and add macos-13. At this
time, macos-latest is macos-14.

.github/workflows/main.yml

@@ -237,7 +237,7 @@ jobs:
       RUSTFLAGS: --cfg rustix_use_experimental_features
     strategy:
       matrix:
-        build: [ubuntu, ubuntu-20.04, i686-linux, aarch64-linux, powerpc64le-linux, riscv64-linux, s390x-linux, arm-linux, ubuntu-stable, i686-linux-stable, aarch64-linux-stable, riscv64-linux-stable, s390x-linux-stable, powerpc64le-linux-stable, arm-linux-stable, ubuntu-1.63, i686-linux-1.63, aarch64-linux-1.63, riscv64-linux-1.63, s390x-linux-1.63, powerpc64le-linux-1.63, arm-linux-1.63, macos-latest, macos-12, windows, windows-2019, musl]
+        build: [ubuntu, ubuntu-20.04, i686-linux, aarch64-linux, powerpc64le-linux, riscv64-linux, s390x-linux, arm-linux, ubuntu-stable, i686-linux-stable, aarch64-linux-stable, riscv64-linux-stable, s390x-linux-stable, powerpc64le-linux-stable, arm-linux-stable, ubuntu-1.63, i686-linux-1.63, aarch64-linux-1.63, riscv64-linux-1.63, s390x-linux-1.63, powerpc64le-linux-1.63, arm-linux-1.63, macos-latest, macos-13, windows, windows-2019, musl]
         include:
           - build: ubuntu
             os: ubuntu-latest
@@ -410,8 +410,8 @@ jobs:
           - build: macos-latest
             os: macos-latest
             rust: stable
-          - build: macos-12
-            os: macos-12
+          - build: macos-13
+            os: macos-13
             rust: stable
           - build: windows
             os: windows-latest