bug#64855: 30.0.50; ERC 5.6: Make scrolltobottom less erratic

[J.P.]

Due to various outstanding issues related to this feature, I've decided
to abandon almost all window twiddling in `post-command-hook' because
blacklisting and whitelisting various commands was becoming a sad game
of whack-a-mole. Rather, I think it's best if we restrict this option's
availability to Emacs 28+ and leverage `read-minibuffer-restore-windows'
to keep windows stable when executing extended commands. I also think it
makes sense to consolidate knobs and have `erc-scrolltobottom-all'
subsume the do-little `erc-scrolltobottom-relaxed' as a third value
state (boolean + the symbol `relaxed'). Please see attached changes.
Thanks.

0001-5.6-Skip-restore-post-minibuf-read-in-erc-scrolltobo.patch
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