Push deprecation of std.exception.enforceEx

[wilzbach]

In #6086 we prepared enforce to be a super set of enforceEx and thus allow a deprecation of it.

See also: https://dlang.org/changelog/2.079.0.html#std-exception-enforce

However, if enforce can't fully replace enforceEx, then it needs to stick around for at least one more release so that we can have a release where enforce can fully replace enforceEx.

This has happened, so we finally can start the deprecation rodeo.

The targetted deprecation version is 2.084 as it's marked as a legacy symbol in the docs for quite a while now.