Copy in release notes for last maintenance releases into HISTORY

Copy in release notes for last maintenance releases into HISTORY

Copy in release notes for last releases:
1.26.2: Iae3ab6fe60b348abf6f2723b2601bd64fb3261ba
1.25.5: I995da41b703eab7b6435ab6745d8513b53c533f5
1.24.6: I05d2c3e6ed8527ba1933558f9636a3beb2d6bd80
1.23.13: I5080cf9fa699c59ee5d2bb2ae9bbbb09fa262e41

Remove Compatibility and Upgrading section from 1.25, previous releases
does not include this section, too.

Update header structure:
- Added missing header
- Increase h2 to h3 when needed
- Decrease h2 to h1 when needed

Change-Id: I37a1134c539bdcb728816fe9825c56de18878d03