All development now on new branch "main"

[julian-smith-artifex-com]

We have moved all PyMuPDF development to a single new branch main.

The new branch main is now the repository's default branch; so for example a
git clone will result in a checkout of main, not master.

The old branches 1.21 and master will no longer be used.

Reasons for doing this:

• The use of a separate release branch wasn't really helping PyMuPDF
  development, because we don't generally release patches for old releases.

• We've recently started testing the latest PyMuPDF in git with different
  versions of MuPDF, using a small number of #if...#endif constructs in the
  code to cope with the occasional changes to the MuPDF API.

  This has proved to be much simpler in practise than, for example, having
  a separate branch of PyMuPFD for the current MuPDF 1.xx release and for
  MuPDF's master branch.

• Github doesn't make it easy for workflows to be on a non-default branch. So
  we previously had (most) workflows on master, even though all code/docs
  work was on branch 1.21. This was confusing to work with.

• We are using main instead of master for the reasons described in
  https://github.com/github/renaming.

The new branch main is a copy of the old 1.21 branch, with some new commits
added:

• A commit to revert the small number of unused commits to workflows that were
  made on branch 1.21 after it branched off master.

• Cherry-picks of all workflow-related commits on master since 1.21 was
  created.

Thus branch main now contains all of the useful code, docs and workflows from
the old branches 1.21 and master.

Notes about changing history:

• The git history of code/docs commits (from branch 1.21) is unchanged.

• The history of workflow scripts etc (from master) is changed in git.

Moving to a single branch will always involve changing history somewhere; we've
choosen to do this for the relatively small number of workflow commits, not
the large number of code/docs commits.

Here's an ascii-art description of the branches, oldest commits first.
(f0b0a93 is the git sha where branch 1.21 was created from master.):

    branch 1.21         master          main
    -----------         ------          ----------
    ...                 ...             ...
    f0b0a93a            f0b0a93a        f0b0a93a
    code-1              workflow-1      code-1
    workflow-unused     code-unused     workflow-unused
    code-2              workflow-2      code-2
    code-3              workflow-3      code-3
    ...                                 ...
    code-N                              code-N
                                        ~workflow-unused (revert)
                                        workflow-1
                                        workflow-2
                                        workflow-3