[ci] update MSRV to 1.46.0

[notmandatory]

Description

This change increases BDKs minimum supported rust version to 1.46.0. This will fix a problem with a recently updated version of hyper which breaks our CI build with the current MSRV 1.45.0.

Notes to the reviewers

Since rust 1.45.0 is already five versions back I think it's reasonable to increment it now. Our other option is to pin the hyper version to 0.14.4 (see #316) but that change breaks our check-wasm CI job. I don't think we need to go down that rabbit hole since we shouldn't have any users who can't update to at least 1.46.0.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

[notmandatory]

If everyone is on board with this change then I'll change the required checks to 1.46.0 instead of 1.45.0.

[LLFourn]

ACK

[notmandatory]

Moved back to DRAFT until we can get #316 done and a 0.5.1 release out. Then we can re-evaluate this change for a future release.

[afilini]

I would rebase this on #318 and revert the pinning of hyper as well since it won't be needed on 1.46

[notmandatory]

I also updated the github required checks for master branch from 1.45.0 to 1.46.0.