Fancy regex has a max backtracking limit which defaults to 1,000,000
backtracks. This avoids spinning the CPU forever in the case that a
match is taking a long time (though does mean that some matches may be
missed).

Unfortunately the verison we depended on causes an infinite loop when
the backtracking limit is hit
(https://github.com/fancy-regex/fancy-regex/issues/137), so we got the
worse of both worlds: matches were missed *and* we spun the CPU forever.

Updating fixes this.

Excitingly regex may gain support for lookarounds
(https://github.com/rust-lang/regex/pull/1315), which will make
fancy-regex much less load bearing.

Closes #43821

Release Notes:

- Fix a bug where search regexes with look-around or backreferences
could hang
  the CPU. They will now abort after a certain number of match attempts.