Bump MSRV to 1.60.0

[str4d]

Rationale:

• Weak dependency features (so we can conditionally enable ff_derive features based on ff features without force-enabling ff_derive).

1.60.0 was released April 7, so we won't be doing this before July at the earliest. ff 0.12 will have MSRV 1.56 (#74).

[koivunej]

Would suggest considering specifiying rust-version = "1.60" in Cargo.toml but that's a growing list of:

• gh actions workflow
• rust-toolchain
• README.md

Perhaps README.md mention could be dropped off in favor of cargo metadata?

[str4d]

I will be doing that for 1.56, as that's the first release that supports it.

[str4d]

We are about to cut 0.12.1 (#92), after which we will be making breaking changes that necessitate an 0.13. I am contemplating making this MSRV bump in 0.13, but will check what the MSRVs are in downstream crates first.