refactor(ci): build on macOS 12 instead of 10.15

GitHub is deprecating their macOS 10.15 VMs [1]. Switch to the latest
supported version of macOS (12) for builds.

-Wfallback isn't supported by Clang 13 and newer, so drop it from the
warning set.

[1] actions/runner-images#5583

.github/workflows/build-and-test-plugin-vscode.yml

@@ -14,7 +14,7 @@ jobs:
       fail-fast: false
       matrix:
         os:
-          - {runs_on: macos-10.15, name: "macOS x86_64", platform_arch: "darwin-x64", test: true, CC: /usr/local/opt/llvm@13/bin/clang, CXX: /usr/local/opt/llvm@13/bin/clang++, CFLAGS: "-isystem /usr/local/opt/llvm@13/include -isystem /usr/local/opt/llvm@13/include/c++/v1 -mmacosx-version-min=10.9 -D_LIBCPP_DISABLE_AVAILABILITY", LDFLAGS: "-L/usr/local/opt/llvm@13/lib -mlinker-version=278 -nostdlib++ /usr/local/opt/llvm@13/lib/libc++.a /usr/local/opt/llvm@13/lib/libc++abi.a"}
+          - {runs_on: macos-12, name: "macOS x86_64", platform_arch: "darwin-x64", test: true, CC: /usr/local/opt/llvm@13/bin/clang, CXX: /usr/local/opt/llvm@13/bin/clang++, CFLAGS: "-isystem /usr/local/opt/llvm@13/include -isystem /usr/local/opt/llvm@13/include/c++/v1 -mmacosx-version-min=10.9 -D_LIBCPP_DISABLE_AVAILABILITY", LDFLAGS: "-L/usr/local/opt/llvm@13/lib -mlinker-version=278 -nostdlib++ /usr/local/opt/llvm@13/lib/libc++.a /usr/local/opt/llvm@13/lib/libc++abi.a"}
           - {runs_on: stracle-macos-aarch64, name: "macOS AArch64", platform_arch: "darwin-arm64", test: true, CFLAGS: "-mmacosx-version-min=11.0"}
             # NOTE(strager): We test on Linux x86_64 in the build-extension job.
           - {runs_on: ubuntu-latest, name: "Linux x86_64", platform_arch: "linux-x64", test: false, docker_container: "ghcr.io/quick-lint/quick-lint-js-github-builder:v1", CC: gcc-8, CXX: g++-8, LDFLAGS: "-static-libgcc -static-libstdc++"}