[libc++][CI] Updates Docker image.

- Updates the image to use Ubuntu Jammy.
- Installs GCC-12 as preparation to migrate to that GCC version.

NOTE: This is a re-application of f2f0dba8, which was reverted
in 2b5e3ef8 due to an issue with the CI nodes. The CI nodes have
since then been updated and this appears to be fine.

Differential Revision: https://reviews.llvm.org/D126666