Revert dip1000 deprecations for 2.100

[dkorpel]

Disable deprecation warnings introduced by #12578. Doesn't completely remove the supporting code, so re-enabling should be easy.

The release for 2.100 came faster than expected, and lots of things aren't ready yet: Documentation is still being written, better safety error messages (#13957) haven't landed, many deprecation warnings for would-be dip1000 errors are missing, and deprecation warnings it does give may be false positives since inference of scope / return is currently broken without -preview=dip1000.

@WalterBright @atilaneves I propose to move this change to 2.101, because I'm afraid rushing the transition is only giving confusion to users.

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