14.0 relnotes: cleanups

• Add "14.0" section heading after 13.1/13.2 section, clarifying which changes are in which release.
• Move igc into 13.1/13.2 section.
• Remove redundant PIE item, now that it is in 13.1/13.2 section, but keep "sponsored by".
• Move MANA into cloud section.

Differential Revision: https://reviews.freebsd.org/D42668