ERC: Track: Rewrite 'erc-track-find-face' as 'erc-track-select-mode-line-face'

ERC: Track: Rewrite 'erc-track-find-face' as 'erc-track-select-mode-line-face'

* lisp/erc/erc-track.el (erc-track-find-face): Declare obsolete and
rewrite as 'erc-track-select-mode-line-face', changing the function
arguments, so that it is very clear what the current algorithm is.
No functional changes.  Performance improvements.  Clarify the
documentation and remove the part on some faces being lists, which
clearly does not apply.
(erc-track-modified-channels): Replace calls to 'erc-track-find-face'
with calls to 'erc-track-select-mode-line-face', preserving the
existing behavior.
(erc-modified-channels-alist): Change the reference to
'erc-track-select-mode-line-face' in the documentation following the
rename.
* etc/NEWS: Announce the change.

Co-authored-by: Amin Bandali <bandali@gnu.org>