Decide on MSRV

[bjoernQ]

See #787

I like the CURRENT_VERSION - 1 idea and with 7b45dca that should be easy to implement

[jessebraham]

Is there any reason not to just use 1.67.0, which is currently required for the async example, and remove the distinction there altogether? I don't think there are really any new language features we want to take advantage of (correct me if I'm wrong) so giving some headroom would be nice.

[bjoernQ]

Also fine for me. Don't have a strong opinion on this other than having to pin a (transitive) dependency because of the MSRV is not a good thing

[MabezDev]

I don't think we should blindly upgrade to CURRENT_VERSION - 1 whenever its available, but if there is a feature we want we can just go ahead and update provided its at least one version behind from current stable. We could also of course scrap that idea and just address it on a case-by-case basis.

[bjoernQ]

#779 additionally requires toml_edit to be pinned in esp32c6-lp-procmacros

If we need to bump MSRV we should make sure it's at least CURRENT_VERSION - 1 or older.
But we won't bump the MSRV just because we can

[bjoernQ]

The only thing left to do to close this issue is bump to MSRV 1.67 and unpin the toml_edit dependency