Figure out new MSRV target >= 1.71.0

[tnull]

As rustls recently bumped their MSRV to 1.71, we want to figure out a new MSRV target that at least covers this to make sure our users leaning on MSRV can always build with the latest rustls (which might get security-critical patches).

To this end we recently started a conversation over at bitcoindevkit/bdk#1750 as we'll probably want to figure out a common MSRV target across crates in the space.