Add support for macOS 14.

Drop support for macOS 11.

Fixes #453.

Change-Id: Id73df696b5610246ee8844e5065e10620423f849
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62551
Reviewed-by: Paul Wankadia <junyer@google.com>
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>