macOS: raise deployment target to 10.13

```
error: 'visit<(lambda at ...include/openPMD/backend/Attribute.hpp:239:9),
              std::variant<...> &>'
is unavailable: introduced in macOS 10.13
```

`std::filesystem` needs macOS 10.15:
```
share/openPMD/thirdParty/toml11/toml/parser.hpp:2166:65:
error: 'path' is unavailable: introduced in macOS 10.15
basic_value<Comment, Table, Array> parse(const std::filesystem::path& fpath)
                                                                ^
/Applications/Xcode_13.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.0.sdk/usr/include/c++/v1/filesystem:902:24:
note: 'path' has been explicitly marked unavailable here
class _LIBCPP_TYPE_VIS path {
                       ^
```

.github/workflows/macos.yml

@@ -27,10 +27,11 @@ jobs:
         brew install python || true
         set -e
     - name: Build
-      env: {CXXFLAGS: -Werror -Wno-deprecated-declarations, MACOSX_DEPLOYMENT_TARGET: 10.9}
+      env: {CXXFLAGS: -Werror -Wno-deprecated-declarations -DTOML11_DISABLE_STD_FILESYSTEM, MACOSX_DEPLOYMENT_TARGET: 10.13}
       # C++11 & 14 support in macOS 10.9+
       # C++17 support in macOS 10.13+/10.14+
       #   https://cibuildwheel.readthedocs.io/en/stable/cpp_standards/#macos-and-deployment-target-versions
+      # std::filesystem needs macOS 10.15
       run: |
         share/openPMD/download_samples.sh build
         chmod u-w build/samples/git-sample/*.h5