MAINTAINERS: Update pstore maintainers

Update pstore to better reflect reality of active contributors:

- Remove Anton and Colin (thank you for your help through the years!)
- Move Tony to Reviewer
- Add Guilherme as Reviewer
- Add mailing list
- Upgrade to Supported

Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Acked-by: Colin Cross <ccross@android.com>
Link: https://lore.kernel.org/r/20221011200112.731334-6-keescook@chromium.org