ECDSA JRuby fix

[rchekaluk]

This PR addresses JRuby failures noted here. Tested under rubies ruby-2.5.8, ruby-2.6.6, ruby-2.7.6, ruby-3.0.0, ruby-3.1.2, ruby-3.2.2, ruby-3.3.0-preview1, jruby-9.4.2.0, jruby-9.4.3, jruby-9.4.4.0

There are a couple of layered issues when using JRuby:

• jruby-openssl does not suffer from "OpenSSL::PKey::PKeyError: pkeys are immutable on OpenSSL 3.0" like ruby-openssl, thus sshkey can continue using the original technique for determining public key
• JRuby does not implement point_conversion_form, so note that explicitly in case it is used in the future
• JRuby uses curve name "prime256v1" not its alias "secp256r1", so convert that explicitly in case it is used in the future

[bensie]

Awesome work. Thank you!