Ignore date stamps completely in erc-track

Ignore date stamps completely in erc-track

* etc/ERC-NEWS: Mention that date stamps no longer optionally affect
the mode line.  Also mention but discourage new variable
'erc-stamp-prepend-date-stamps-p'.
* lisp/erc/erc-stamp.el (erc-stamp-prepend-date-stamps-p): New
variable, an escape hatch to allow date stamps to once again be
prepended to messages.
(erc-insert-timestamp-left-and-right): Don't insert stamps as
independent messages when legacy support flag
`erc-stamp-prepend-date-stamps-p' is non-nil.
* lisp/erc/erc-track.el (erc-track--skipped-msgs): New internal
variable.
(erc-track-modified-channels): In previous versions, a date stamp
attached to a message for an IRC command in `erc-track-exclude-types'
would have no effect on the mode line.  That they were able to
otherwise was probably a bug.  Regardless, this distinction was lost
for the worse after date stamps became independent messages with
c68dc7786fc "Manage some text props for ERC insertion-hook members".
To sidestep this regression, the `track' module will ignore date
stamps completely from now on.  Thanks to Corwin Brust for spotting
this.
* test/lisp/erc/erc-scenarios-stamp.el
(erc-scenarios-stamp--left/display-margin-mode): Remove redundant
binding.
(erc-scenarios-stamp--legacy-date-stamps): New test.  (Bug#60936)