Respect the configured MSRV in implicit_saturating_sub's if x != 0 { x -= 1 } rewrite

[shulaoda]

Fixes #17403

implicit_saturating_sub rewrites if x != 0 { x -= 1 } into x = x.saturating_sub(1). saturating_sub is only usable in a const fn since Rust 1.47, so in a const fn with a lower configured MSRV the rewrite does not compile.

#16309 already added this MSRV check to the if a >= b { 0 } else { b - a } path, but the condition path (if x != 0 { x -= 1 }) was left unguarded. This threads the MSRV into that path and applies the same check, so the lint stays quiet when the suggested saturating_sub would be too new for the const context.

The lint reads the MSRV but was missing from the list of MSRV affected lints in the configuration docs, so this registers it there as well.

changelog: [implicit_saturating_sub]: respect the configured MSRV in const contexts for the if x != 0 { x -= 1 } rewrite

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