PKCS#5 version 1.5 padding is banned for key agreement purposes from January 2024 as part of SP 800-131Ar2.

Since it is now a soft transition and if we submit before then, we're okay until we need to update the FIPS provider.

I.e. we have to do this eventually.

The two obvious approaches are:

1. an indicator that is set if this mode is used
2. condition out the relevant bits of code

There is a third which is: do both.