Rollup merge of #84120 - workingjubilee:stabilize-duration-max, r=m-ou-se

Rollup merge of #84120 - workingjubilee:stabilize-duration-max, r=m-ou-se

Stabilize Duration::MAX

Following the suggested direction from https://github.com/rust-lang/rust/issues/76416#issuecomment-817278338, this PR proposes that `Duration::MAX` should have been part of the `duration_saturating_ops` feature flag all along, having been

0. heavily referenced by that feature flag
1. an odd duck next to most of `duration_constants`, as I expressed in https://github.com/rust-lang/rust/issues/57391#issuecomment-717681193
2. introduced in #76114 which added `duration_saturating_ops`

and accordingly should be folded into `duration_saturating_ops` and therefore stabilized.

r? `@m-ou-se`