crypto/rsa: 4096 bit keys are not generated with BoringCrypto

[FiloSottile]

https://go.dev/cl/423362 updated the BoringCrypto module to one that supports RSA 4096 bit keys, and #41147 removed the limitation in crypto/x509, but crypto/rsa still has a check that only uses BoringCrypto for 2048 and 3078 bit keys, so 4096 bit keys are generated with the pure Go implementation.

/cc @golang/security @rsc

[FiloSottile]

@gopherbot please open a Go 1.20 backport issue for this.

Go 1.20 updated the BoringCrypto module and started using BoringCrypto for RSA 4096 bit keys in crypto/x509, but not in GenerateKey. The two are not necessarily correlated (most X.509 keys are loaded from disk, not generated) but using unverified crypto unexpectedly is not great, and the fix is simple.

/cc @rsc @golang/security

[gopherbot]

Backport issue(s) opened: #58927 (for 1.20).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.

[gopherbot]

Change https://go.dev/cl/474515 mentions this issue: crypto/rsa: use BoringCrypto for 4096 bit keys