CI: macOS 11.0+ (#1486)

We do not have the runners anymore to deploy and test macOS 10.15,
so we bump our tests to 11.0+, too.

Technically, we could build & deploy a bit longer on GH actions
`macos-11` for `10.15` (Catalina), but since it is unmaintained by
Apple/end-of-life already and macOS-11 is the oldest still maintained
OS by Apple, we do also stop support for it.

At the time of writing, old and maintained are:
- macOS 11, Big Sur
- macOS 12, Monterey

latest is: macOS 13, Ventura - and in preview is macOS 14, Sonoma.

This also unifies our arm64/aarch64 (M1/M2) requirements, which are
macOS 11.0+ as well.

.github/workflows/macos.yml

@@ -28,10 +28,11 @@ jobs:
         python3 -m pip install -U mpi4py numpy pandas
         set -e
     - name: Build
-      env: {CXXFLAGS: -Werror, MACOSX_DEPLOYMENT_TARGET: 10.15}
+      env: {CXXFLAGS: -Werror, MACOSX_DEPLOYMENT_TARGET: 11.0}
       # 10.14+ due to std::visit
       # 10.15+ due to std::filesystem in toml11
       #   https://cibuildwheel.readthedocs.io/en/stable/cpp_standards/#macos-and-deployment-target-versions
+      # 11.0+ for arm64/aarch64 (M1/M2) builds
       run: |
         share/openPMD/download_samples.sh build
         cmake -S . -B build \
@@ -49,16 +50,17 @@ jobs:
     steps:
     - uses: actions/checkout@v3
     - name: Install
-      env: {MACOSX_DEPLOYMENT_TARGET: 10.14}
+      env: {MACOSX_DEPLOYMENT_TARGET: 11.0}
       run: |
         set +e
         python3 -m pip install -U numpy pandas
         set -e
     - name: Build
-      env: {CXXFLAGS: -Werror -DTOML11_DISABLE_STD_FILESYSTEM, MACOSX_DEPLOYMENT_TARGET: 10.14}
+      env: {CXXFLAGS: -Werror, MACOSX_DEPLOYMENT_TARGET: 11.0}
       # 10.14+ due to std::visit
-      # std::filesystem in toml11 needs macOS 10.15
+      # 10.15+ due to std::filesystem in toml11
       #   https://cibuildwheel.readthedocs.io/en/stable/cpp_standards/#macos-and-deployment-target-versions
+      # 11.0+ for arm64/aarch64 (M1/M2) builds
       run: |
         share/openPMD/download_samples.sh build
         cmake -S . -B build \