Split changelog file into separate files

Description

Every release makes our CHANGELOG file havier.
At the moment it is 258Kb of size (the last release is 9.2.6).

Proposal

1. Split CHANGELOG file into separate files per major release. Like CHANGELOG-8.x, CHANGELOG-9.x, CHANGELOG-10.x, etc.
2. Move all the content from the changelogs/archive.md to separate changelog files and remove this file.

At the end we will have the structure:

• /changelogs/CHANGELOG-8.x
• /changelogs/CHANGELOG-9.x
• /changelogs/CHANGELOG-10.x
• etc

Splitting will let users open changelogs faster.
There will be no need to load all the entries of releases an user's not interested in.

release-tools should be changed too as part of this proposal.

/cc @rymai @DouweM @rspeicher @smcgivern @stanhu