RUST-1375 Update MSRV policy

[abr-egn]

RUST-1375

This:

• gets rid of the "will rarely be increased" phrase; we tried to hew to this by jumping through hoops to do it as little as possible but literally no other crates I could find make this promise. Likewise "if it ever is" was long since dead verbiage 🙂.
• Sets the six month rolling window. Of those crates that do provide a written policy, this seems to be a little on the conservative side of normal; only rustls sets a longer window and most do a shorter one.

I considered defining the window in terms of a set number of releases instead of a time period (both are common in the crate ecosystem) and went with time period because I find that a little easier to personally reason about.

[isabelatkinson]

lgtm!