PR #10381 added an Unreleased section above 26.5.0 to capture work
landing after RC1. Cutting RC2 from the head of main means those
entries are part of the 26.5.0 binary, so the release notes for the
26.5.0 tag must include them. This commit moves the entries from
Unreleased into the matching 26.5.0 subsections (Breaking Changes,
Bug fixes, Additions and Improvements) and leaves Unreleased as an
empty placeholder for post-26.5.0 work.

Signed-off-by: jflo <justin+github@florentine.us>