Update lower bound for RSA key sizes

[ptoffy]

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

Checklist

• I've run tests to see all new and existing tests pass
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• 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

[Lukasa]

Thanks for this change! However, as written I don’t think we should take it.

RSA support is not provided for new uses, but for compatibility with old ones. To that compatibility end, some support for weaker key sizes is likely to be necessary.

However, I do think we could make the case for this change if we introduced an explicitly marked unsafe or unchecked initializer that reinstated the current bounds. Are you open to that?

[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