update list of allowed ciphers

[markdboyd]

Changes proposed in this pull request:

• update list of allowed ciphers that are incorrectly flagged as false positives based on NIST guidance

Security Considerations

Even though these ciphers are using RSA cryptography that is generally being phased out throughout the industry, we are updating our list of allowed ciphers to inform customers that we do not consider them as "weak" given that:

• They use Amazon's internal s2n-tls implementation that mitigates some known attacks on RSA ciphers
• NIST guidance on transitioning away from RSA ciphers explicitly identifies these ciphers as allowed while RSA ciphers are still in use (page 52)