Rename as many as possible "master" occurrences to "main"

[stronk7]

That includes:

• All those making reference to our (old) master branch.
• All those, part of a URL (usually git repository) also renamed when verified that they work.

We have kept apart a few corresponding to:

• Other "master" meanings, unrelated with the branches.
• Some old release notes from 2.x versions.
• general/development/tools/mdk.md (not sure what's changed there)
• general/community/plugincontribution/master2main.md, because it explains exactly the move from master to main.

Also, a few things found, worth being considered:

1. Suggestion: Maybe we could add some new test in order to detect any URL using the old "master" branch and failing on that, possible candidates are URLs like:
   • https://github.com/moodle/moodle/blob/master/....
   • https://git.in.moodle.com/moodle/moodle/-/blob/master/...
2. While working on this I saw that there were a few links to the wiki docs of pages that already have been migrated. I've fixed them as part of the commit, but there should be some process detecting them at regular intervals at very least. The changes I applied were:
   • https://docs.moodle.org/dev/Message_API => ./apis/core/message/index.md
   • https://docs.moodle.org/dev/Cache_API => ./apis/subsystems/muc/index.md
3. Related with the previous point, are the internal links like the ones above checked? I ask because I did a typo in one of the 2 conversions above and the pre-commit hook did not detect it... maybe only GH workflows do that?
4. I just saw the the jsdocs api docs are still using "master" instead of "main", surely we should make the switch there.

And that's all. I've performed one more search and all the remaining cases of "master" are justified, as explained above.

Again, please, don't forget about general/development/tools/mdk.md I've left it unmodified because it was not clear to me which "master" occurrences have to / can be replaced and which ones cannot. Friendly pinging to @junpataleta about that.

Ciao :-)

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