Major update for maintainer-pgp-guide

commit e4412739 ("Documentation: raise minimum supported version of binutils to 2.25")
commit 67fe6792 ("Documentation: stable: Document alternative for referring upstream commit hash")
commit 8763a30b ("docs: deprecated.rst: Add note about DECLARE_FLEX_ARRAY() usage
commit 2f993509 ("docs: process/5.Posting.rst: clarify use of Reported-by: tag")
commit a31323be ("timers: Update the documentation to reflect on the new timer_shutdown() API")
Signed-off-by: Federico Vaga <federico.vaga@vaga.pv.it>
Link: https://lore.kernel.org/r/20230319134624.21327-1-federico.vaga@vaga.pv.itSigned-off-by: Jonathan Corbet <corbet@lwn.net>