Update changelog up to release v2.6.3

[agoscinski]

This includes the updates of the changelog up to the v2.6.3 release with several fixes:

• Fixes the urls of commits in v2.6.2 release
• Adds missing date in v2.6.3 release

Furthermore a link of the changelog to pyproject.toml has been added. This makes the url to the changelog appear for releases on pypi.

[agoscinski]

The fixes the urls of commits in v2.6.2 release have been already done in another PR, so I can remove it from the commit message