Make WIP prefixes case insensitive, e.g. allow `Draft` as a WIP prefix (#19780) ...

Make WIP prefixes case insensitive, e.g. allow `Draft` as a WIP prefix (#19780) (#19811)

Backport #19780

The issue was that only the actual title was converted to uppercase, but
not the prefix as specified in `WORK_IN_PROGRESS_PREFIXES`. As a result,
the following did not work:

    WORK_IN_PROGRESS_PREFIXES=Draft:,[Draft],WIP:,[WIP]

One possible workaround was:

    WORK_IN_PROGRESS_PREFIXES=DRAFT:,[DRAFT],WIP:,[WIP]

Then indeed one could use `Draft` (as well as `DRAFT`) in the title.
However, the link `Start the title with DRAFT: to prevent the pull request
from being merged accidentally.` showed the suggestion in uppercase; so
it is not possible to show it as `Draft`. This PR fixes it, and allows
to use `Draft` in `WORK_IN_PROGRESS_PREFIXES`.

Fixes #19779.

Co-authored-by: zeripath <art27@cantab.net>