Add back support for SHA224 based sig algs

[mattcaswell]

This used to work but was inadvertently removed as part of the TLSv1.3 work. This adds it back.

Fixes #3633

Note: It seems we are offering disallowed sig algs in a TLSv1.3 compliant ClientHello (including sha1, and now sha224). Draft-20 says this:

  These algorithms are deprecated as of
  TLS 1.3.  They MUST NOT be offered or negotiated by any
  implementation.  In particular, MD5 [SLOTH], SHA-224, and DSA MUST
  NOT be used.

Probably we should only offer sha1/sha224 if TLSv1.3 has been disabled. That would be a different PR though.

Checklist

• documentation is added or updated
• tests are added or updated

[kaduk]

Note: It seems we are offering disallowed sig algs in a TLSv1.3 compliant ClientHello (including sha1, and now sha224).

I raised this in #3326 and it was pointed out that complying with the 1.3 spec would be a regression in a point release, so we ended up only omitting the forbidden algorithms in a 1.3-only ClientHello. (N.B. #3326 is still open.)

[mattcaswell]

I raised this in #3326 and it was pointed out that complying with the 1.3 spec would be a regression in a point release, so we ended up only omitting the forbidden algorithms in a 1.3-only ClientHello. (N.B. #3326 is still open.)

Ahh, right. I thought the topic seemed a little familiar.

[mattcaswell]

Pushed. Thanks.