Bump default Xcode version to 12.4. (#1096)
Bump default Xcode version to 12.4. (#1096)

This doesn't affect Bazel due to https://github.com/bazelbuild/bazel/commit/caf13559e367da9c791cc5e559d2970400d5478b.

It has the risk of breaking downstream projects, but we'll have to fix or disable them then (remaining on an older Xcode version is not viable).
bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration