With TUI as an external process oldtests no longer involve threads, so
TSAN isn't useful. Meanwhile functionaltests may involve threads.

The default merge branch is unreliable when trying to determine number
of commits in a PR. Using the HEAD branch of the PR removes this
ambiguity.

Problem:
Nvim has Lua but the "nvim" CLI can't easily be used to execute Lua
scripts, especially scripts that take arguments or produce output.

Solution:
- support "nvim -l [args...]" for running scripts. closes #15749
- exit without +q
- remove lua2dox_filter
- remove Doxyfile. This wasn't used anyway, because the doxygen config
  is inlined in gen_vimdoc.py (`Doxyfile` variable).
- use "nvim -l" in docs-gen CI job

Examples:

    $ nvim -l scripts/lua2dox.lua --help
    Lua2DoX (0.2 20130128)
    ...

    $ echo "print(vim.inspect(_G.arg))" | nvim -l - --arg1 --arg2
    $ echo 'print(vim.inspect(vim.api.nvim_buf_get_text(1,0,0,-1,-1,{})))' | nvim +"put ='text'" -l -

TODO?
  -e executes Lua code
  -l loads a module
  -i enters REPL _after running the other arguments_.

ci: remove "needs:response" label if author responds

The default behavior of the stale action is to indiscriminately remove
the `needs:response` label for any activity whatsoever, from anyone. The
other option is to turn it off completely, meaning the maintainers needs
to manually remove the label themselves when the author responds for an
issue to not close automatically. Neither of these behaviors are useful
to us.

This was set explicitly to ubuntu.22.04 as ubuntu-latest pointed to
ubuntu.20.04, and we needed 22.04 to have a new enough doxygen version
for this job to work. Now that ubuntu-latest points to 22.04 this
workaround is no longer needed.

asan_symbolize-14 gives a deprecation as it relies on outdated python
features. We can safely stop using asan_symbolize as it's only needed
for special cases such as cross compilation which we don't have to worry
about.