docs(rpiv-pi): move the all-advisory citation-floor entries to [Unreleased]

d3f69b4c's changelog text was drafted while the section it amended was
still [Unreleased], but landed after the 2.6.3 promotion — so [2.6.3]
described behavior that shipped after it. Restore [2.6.3] to the tiered
citation floor it actually shipped and record the all-advisory demotion,
resolver gap fixes, and coverage-floor fixes under [Unreleased].