Define PSA_WANT_xxx_KEY_PAIR_yyy step 2/DH

[mpg]

Same as #7771, except for DH, and DH does not have DERIVE, and has not backwards compat concerns, being new in the next release.

Edit: also, since this is the last one in the series:

• double-check that no KEY_PAIR_LEGACY symbols remains anywhere in the source tree
• amend ChangeLog.d/Define-PSA_WANT_KEY_TYPE_xxx_KEY_PAIR_yyy.txt to also mention that code size can be saved by disabling GENERATE or DERIVE when they're not needed.

Depends on: #7614.