Remove macos-11 from CI

Remove macos-11 from CI

Recent build failure on os-zoo reports:
A brownout will take place on June, 8:00 AM – 2:00 PM EST to raise awareness of the upcoming macOS-11 environment removal.

It appears that github is retiring macos-11, so we may as well remove it
to prepare

Fixes #24739

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24744)