Return to stable Rust when 1.65 is released

[zslayton]

Generic associated types (GATs) are being stabilized in v1.65, due out in November.

PR #442 switched our CI builds to beta so we could leverage GATs in the run up to 1.65. When it's released, we need to switch back to stable. We should also specify a Minimum Supported Version of Rust (MSRV, see rust-clippy #6097 for discussion) for the crate.