t/lib-httpd: bump required apache version to 2.4

t/lib-httpd: bump required apache version to 2.4

Apache 2.4 has been out since early 2012, almost 11 years. And its
predecessor, 2.2, has been out of support since its last release in
2017, over 5 years ago. The last mention on the mailing list was from
around the same time, in this thread:

  https://lore.kernel.org/git/20171231023234.21215-1-tmz@pobox.com/

We can probably assume that 2.4 is available everywhere. And the stakes
are fairly low, as the worst case is that such a platform would skip the
http tests.

This lets us clean up a few minor version checks in the config file, but
also revert f1f2b45be0 (tests: adjust the configuration for Apache 2.2,
2016-05-09). Its technique isn't _too_ bad, but certainly required a bit
more explanation than the 2.4 version it replaced. I manually confirmed
that the test in t5551 still behaves as expected (if you replace
"cadabra" with "foo", the server correctly rejects the request).

It will also help future patches which will no longer have to deal with
conditional config for this old version.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>