[GenerateKey] now returns an error if a key of less than 1024 bits is requested.
All Sign, Verify, Encrypt, and Decrypt methods now return an error if used with
a key smaller than 1024 bits. Such keys are insecure and should not be used.
Setting `GODEBUG=rsa1024min=0` or including `//go:debug rsa1024min=0` in a
source file restores the old behavior, but we recommend doing so only in tests,
if necessary. A new [GenerateKey] example provides an easy-to-use standard
2048-bit test key.