chore: Relax MSRV

[tottoto]

Relaxes MSRV which are bumped in #982. As the use of prost-build can be separated to the different build process, it is too strict to set the same MSRV of prost-build to the other crates for users. cargo-hack's --rust-version option can be used to perform checking different MSRVs on the project crates.

[caspermeijn]

I prefer to keep the whole repo at the same MSRV. With the removal of which in #962 the MSRV of the published crates can be lowered to rust 1.65. Would that be enough for your use case?

I suggest to only do a CI build with stable toolchain (so remove the 1.70 toolchain) and to add your proposed MSRV check.

[tottoto]

Since checking MSRV for different crates can be performed easily as this pull request describes, and the crates except for prost-build only depend on very few crates, which have high stability, the value imposing unnecessarily restrictions seems small.

[caspermeijn]

I stick to my position. I want to keep the whole repo at the same MSRV. I believe the MSRV of the crates can be lowered to rust 1.65.

[caspermeijn]

I opened a new PR with only the CI improvement: #1057

[caspermeijn]

The CI improvement from this PR is merged. I won't accept multiple MSRV versions in one repo. If you want to lower the MSRV of the whole repo, you are welcome to reopen.