.. and bump its MSRV to 1.75.
Recently, `rustls` bumped their MSRV to 1.71. As we depend on them and
don't want to continuously pin this security-critical dependency back,
we have no choice left but to bump the MSRV for
`lightning-transaction-sync` to a version >= 1.71, too.
Here, we hence move the `lightning-transaction-sync` tests to a
dedicated script and propose to introduce a secondary MSRV of 1.75.
We chose this particular version, because:
a) it's > 1 year old
b) it provides a buffer to 1.71, i.e., if some crate bumped to a version
> 1.71, there is a chance we don't immediately have to react again
c) it
stabilized `async fn`s in traits (see
https://blog.rust-lang.org/2023/12/21/async-fn-rpit-in-traits.html),
which might become handy for related (BDK) crates, which hopefully will
adopt the same target.
