PR template update & OpenSSL clarification

[baentsch]

• Adds test requirement to PR template (for feature changes)
• Clarifies OpenSSL versions supported/recommended
• Removes oqs-openssl111 and oqs-boringssl from checklist below

• [no] Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• [no] Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in oqs-provider, OQS-OpenSSL, OQS-BoringSSL, and OQS-OpenSSH will also need to be ready for review and merge by the time this is merged.)