Bump MSRV to 1.63

[notmandatory]

The current rust version shipped with debian stable is 1.63 (see bitcoindevkit/bdk#331) so this is a reasonable new MSRV to support in this project. Also this is the MSRV that bdk and rust-lightning will soon support.

[oleonardolima]

I think this got done on #69 and #70, right?

[vladimirfomene]

@oleonardolima yes you are right. Let me close it