Rename `master` branch to `main`

[eamonnmcmanus]

In keeping with recent practice, we should rename the master branch on GitHub to main. This change is well-supported on GitHub.

One notable effect is that we can change the links to the troubleshooting guide that were recently added to some exception messages, so that they look like https://github.com/google/gson/blob/main/Troubleshooting.md#foo instead of https://github.com/google/gson/blob/master/Troubleshooting.md#foo. (The old links will continue to work after the rename, but the new ones will be preferred.)

[eamonnmcmanus]

I've renamed the branch, but we also need #2410 to complete this.

Incidentally, renaming the branch closed a bunch of old PRs that were targeted to the old branch. I think it was only PRs where the originating repo no longer exists.

[Marcono1234]

Incidentally, renaming the branch closed a bunch of old PRs that were targeted to the old branch. I think it was only PRs where the originating repo no longer exists.

For reference, it looks like the following PRs were affected:

• Specify serialization date format type. + Milliseconds and unix date formats. #744
  (looks like an interesting change since in general Gson's java.util.Date formatting is quite messy; but since that pull request is so old, it is unclear if original author would have revived it)
• Add option to let nulls through custom deserializers #852
  (to some extent obsolete due to Fix #1697: Mention that GsonBuilder.registerTypeAdapter makes (de-)serializers null-safe #1704; maybe not such a big impact because nowadays TypeAdapter should be preferred to JsonSerializer and JsonDeserializer)
• Quick fix for Apache MultiValuedMap integration #997
  (probably not something we would have integrated like this)
• Unnecessary unboxing #1436
  (was apparently obsolete)
• Add clear method to JsonArray #1667
  (looks like decision was that method proposed by this pull request will not be added, see discussion there)
• Cleanup code & documentation for exceptions #1671
  (maybe some of the documentation changes would have been nice to have, but nothing critical I guess)
• Added format plugin and formatted whole project #1779
  (probably large amounts of conflicts due to reformatting; also reformatting is tracked by Clean up Gson source code #2284 now, but might still be postponed due to risk of merge conflicts for all PRs?)
• Documentation for collections #1873
  (review comments had not been addressed, and I am not sure if that addition to the user guide would have been that useful)

And apparently three pull requests which don't exist anymore for whatever reason: #1846, #2095, #2368