Update relprep.py in maint-13.5

relprep.py needs some love to work properly in the maint-13.5 branch. I've run into at least the following issues:

• incorrect changelog merging (seems to only grab 14.0 changelogs?)
• should not try to update Go
• incrementals need to ignore the 14.0 channel (iirc we've a similar issue in 14.0 where it adds new 13.5.x versions when 13.5.7 was the watershed)
• should we stop updating the manual, and rollback the current version to the last 13.5 stable version?
• stop generating allowed_addons.json
• stop updating fenix translations

Additionally, the changelog generation script should stop including Linux in Desktop linked issues

/cc @pierov