Upgrade minimum version of macOS support to macOS 10.15 (Catalina).

See prior art:
aa3b976

OSS foundational support matrix suggests that the minimum supported macOS
version has been moved up to macOS 11 (Big Sur).
See: https://github.com/google/oss-policies-info/blob/main/foundational-cxx-support-matrix.md

#test-continuous

PiperOrigin-RevId: 687364677

toolchain/toolchains.bazelrc

@@ -10,7 +10,7 @@ build:linux-s390_64 --config=cross_config --cpu=linux-s390_64
 build:linux-x86_32 --config=cross_config --cpu=linux-x86_32
 build:linux-i386 --config=cross_config --cpu=linux-x86_32
 build:linux-x86_64 --config=cross_config --cpu=linux-x86_64
-build:osx-aarch_64 --config=cross_config --action_env=MACOSX_DEPLOYMENT_TARGET=10.9 --cpu=osx-aarch_64
-build:osx-x86_64 --config=cross_config --action_env=MACOSX_DEPLOYMENT_TARGET=10.9 --cpu=osx-x86_64
+build:osx-aarch_64 --config=cross_config --action_env=MACOSX_DEPLOYMENT_TARGET=10.15 --cpu=osx-aarch_64
+build:osx-x86_64 --config=cross_config --action_env=MACOSX_DEPLOYMENT_TARGET=10.15 --cpu=osx-x86_64
 build:win32 --config=cross_config --cpu=win32
 build:win64 --config=cross_config --cpu=win64