Clarify s2nc/s2nd PQ output

[lrstewart]

Description of changes:

#4700 brought up some questions I suspect future users will have too. The output of s2nc/s2nd doesn't make the relationship between KEM / KEM group / curve clear. To fix that, I switched them to only print the PQ information if PQ is configured, and specifically warn about legacy TLS1.2 vs recommended TLS1.3. Most people using s2nc/s2nd with PQ will probably just want an answer to "am I doing PQ now", so I also made that very clear.

TLS1.2 PQ output:

Legacy TLS1.2 KEM: kyber512r3 (PQ key exchange enabled, but TLS1.3 PQ hybrid key exchange strongly recommended instead. See https://github.com/aws/s2n-tls/blob/main/docs/usage-guide/topics/ch15-post-quantum.md)
Curve: secp256r1
Cipher negotiated: ECDHE-KYBER-RSA-AES256-GCM-SHA384

TLS1.3 PQ output:

KEM Group: SecP256r1Kyber768Draft00 (PQ key exchange enabled)
Cipher negotiated: TLS_AES_128_GCM_SHA256

Without PQ output:

Curve: secp256r1
Cipher negotiated: TLS_AES_128_GCM_SHA256

Testing:

This modifies the output used by the integ tests, so I updated them. Since I added the "PQ key exchange enabled" note for human users, I also updated the tests to check for that note as a nice sanity check.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jmayclin]

I can't quite tell whether "legacy" refers to TLS 1.2 or the KEM. I think it's referring to the KEM in which case I was expecting the linked usage guide to explain why TLS 1.2 PQ was weaker/flawed relative to TLS 1.3, but it doesn't seem to include that information.

Could we either expand on that information here, or expand on it in the linked usage guide?

[lrstewart]

The linked usage guide intentionally doesn't mention TLS1.2 support or list any security policies that would enable it. This comment discussed that choice.

I tried to very briefly explain the recommendation in the output message. While I dislike how long the message is, I prefer only telling people who are already using TLS1.2 PQ over advertising its existence to everyone with the usage guide :)