docs: remove obsolete master synchronization from release process #559

Problem

RELEASING.md instructs maintainers to open a develop → master pull request after each release and merge it without squashing. The master branch is obsolete and has not remained synchronized with develop.

Following this instruction for 2.2.9 produced #558, which attempted to merge 178 commits and change thousands of lines. The PR was closed without merging.

Proposed change

• Confirm that develop is the maintained release branch.
• Remove the obsolete develop → master synchronization section from RELEASING.md, or replace it with the current branch policy.
• Check the surrounding release steps for references that assume master is current.

Acceptance criteria

A maintainer can follow RELEASING.md without opening a broad synchronization PR against an obsolete branch.