Allow editing or removal of out of support branches which cause ci errors

1. you have a  tracker with a bug
2. ages ago someone added a patch for the supported branches at that point in time
3. it sat for ever and didn't land
4. someone picks it up now, and adds the new branches
5. when editing the tracker it doesn't allow you to edit the old branch fields for out of support moodles
6. these branches cannot be found and break the ci

Proposing

• the ci doens't look at these fields in the first place
• or you can edit fields which exist and remove them but you cannot add them back in

We'll often hit this picking up abandoned trackers and despite knowing the problem I constantly forget and have to keep re-figuring out that its a tracker bug not a code bug.

Examples:

https://tracker.moodle.org/browse/MDL-60666

This tracker has a phantom branch:

MOODLE_310_STABLE [branch: MDL-60666-310 | CI Job]

• Error: Unable to fetch information from MDL-60666-310 branch at https://github.com/petersistrom/moodle.