doc: apply sentence case to release doc headers

Our style is to use sentence case for headers and documentation titles.
The documentation for releases uses both sentence case and title case.
This change applies sentence case consistently throughout.

PR-URL: https://github.com/nodejs/node/pull/37349


Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>