Update maturin macOS deployment target to 10.12

```
warning: deployment target in MACOSX_DEPLOYMENT_TARGET was set to 10.9, but the minimum supported by `rustc` is 10.12
warning: `maturin` (lib) generated 1 warning
```

Author: messense

.github/workflows/release.yml

@@ -97,7 +97,7 @@ jobs:
         if: matrix.target == 'x86_64-apple-darwin'
         env:
           DEVELOPER_DIR: /Applications/Xcode.app/Contents/Developer
-          MACOSX_DEPLOYMENT_TARGET: "10.9"
+          MACOSX_DEPLOYMENT_TARGET: "10.12"
         run: |
           # set SDKROOT for C dependencies like ring and bzip2
           export SDKROOT=$(xcrun --sdk macosx --show-sdk-path)