[GitHub] Fix slow sccache install on macOS by upgrading macOS version (#77165)

The "Setup ccache" step on macOS-11 builds takes between 15 to 20 mins,
whereas this step takes a less than 10 seconds on other runners. The
bulk of this time is spent at the "Install sccache" step, where brew
emits warnings like "Warning: You are using macOS 11." and "We (and
Apple) do not provide support for this old version...".

Bumping the version of macOS greatly decreases this cache setup time to
about 20 seconds. Furthermore, it seems like it is speeding up general
build times, too.

It appears that
https://github.com/actions/virtual-environments/issues/5900 has been
resolved or obsoleted, so I do not believe
we need to lock macOS to 11 anymore.