Update minimum macOS to 10.15.

This is per the updated OSS foundational support (https://github.com/google/oss-policies-info/blob/main/foundational-cxx-support-matrix.md) since Chrome has raised its minimum macOS requirements.

PiperOrigin-RevId: 601224079

Author: zhangskz

toolchain/BUILD.bazel

@@ -108,7 +108,7 @@ cc_toolchain_config(
     linker_path = "/usr/tools",
     sysroot = "/usr/tools/xcode_14_0/macosx",
     target_cpu = "aarch64",
-    target_full_name = "aarch64-apple-macosx10.9",
+    target_full_name = "aarch64-apple-macosx10.15",
 )
 
 cc_toolchain_config(
@@ -125,7 +125,7 @@ cc_toolchain_config(
     linker_path = "/usr/tools",
     sysroot = "/usr/tools/xcode_14_0/macosx",
     target_cpu = "x86_64",
-    target_full_name = "x86_64-apple-macosx10.9",
+    target_full_name = "x86_64-apple-macosx10.15",
 )
 
 cc_toolchain_config(