Update lower bound for RSA key sizes

[ptoffy]

Update lower bound for RSA key sizes to 2048 rather than 1024

Checklist

• [x] I've run tests to see all new and existing tests pass
• [x] I've followed the code style of the rest of the project
• [x] I've read the Contribution Guidelines
• [x] I've updated the documentation if necessary (didn't find any documentation to update about this)

Motivation:

Since 2015, NIST recommends a minimum key size of 2048 bits as stated in https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-57pt1r5.pdf This is also stated in the JWA specification (RFC7518 3.3 and 3.5) and in the RSA SSH spec, that's why this PR enforces the use of key sizes >=2048 rather than >=1024 as before.

Modifications:

Update lower bound for RSA key sizes to 2048 rather than 1024, specifically in the RSA key initialisers. Also update tests which use 1024 bits-sized keys and make sure they throw

Result:

Creating RSA keys of sizes <2048 will throw

[ptoffy]

@Lukasa that seems sensible. I've updated the PR with those changes, let me know what you think!

[Lukasa]

@swift-server-bot test this please