docs: add 'Changes since SEP became Final' section to SEP-2243

Preserve SEP-2243 as a historical record instead of editing its body,
and document the HeaderMismatch error code reassignment from -32001 to
-32020 (#2907) in a dedicated section per maintainer feedback.

Also regenerate SEP-2549 docs, whose rendered mdx had drifted from its
source on main, so 'npm run check:seps' passes on PR CI.