Fix MSRV dependencies

[arik-so]

No description provided.

[tnull]

So for one, we already pin MSRV dependencies in ci-tests.sh, not directly in the workflow. If anything, this should be added there.

As previously discussed elsewhere, we could consider pinning rustls back for now, but I think we landed on not doing that as it invites substantial longer-term security risk to our users. So, we likely need to bump the MSRV for lightning-transaction-sync, which in turn might mean we need to move it out of our workspace for now and treat it separately again. (cc @TheBlueMatt)

Also see the related discussion topic I opened on BDK's end: bitcoindevkit/bdk#1750

[arik-so]

Thanks! I drafted this PR for the time being pending. If we decide to move out lightning-transaction-sync it obviously doesn't apply.

[arik-so]

Per our discussion, undrafting this.

[tnull]

Discussed offline: we'll land this for now to fix CI until we move the lightning-transaction-sync out of the workspace and give it its own MSRV.