chore: Bump MSRV to 1.64.0

[epage]

This allows using workspace inheritance which reduces the number of places we need to update on changes. Before, it was easy to remember to update the root manifest and forget the rest.

[djc]

So add some comments? Pushing the MSRV this aggressively for what seems like a pretty minor limitation is a real pain in the ass for downstream maintainers who would prefer to be a little more conservative about supported Rust versions.

[epage]

Comments to help update things is not new to mt. We have a contributing doc and to do MSRV updates, I use comments to tag what needs updating.

Comments and documentation don't reliably scale though. I need tools and processes to keep overhead to a minimum in maintaining my 40+ crates. For example, I automate the release process with cargo-release. The other thing that has worked well is doing directory diffs to update processes between my crates, except when it comes to workspaces. This fills a lot of that gap.

[epage]

Wanted to add that it was also a matter of timing since we relegate MSRV updates to minor versions and I didn't have a clear time table on when the next minor version would be.