Set minimum JDK version back to 8

[tsaarni]

Set the release to JDK8 to maintain compatibility with older JDKs. The version was previously upgraded to JDK15 to support Ed25519. Running with old version led to an error:

.../lib/src/test/java/fi/protonode/certy/TestCredential.java:52: error: cannot find symbol
import java.security.interfaces.EdECPublicKey;
                               ^
  symbol:   class EdECPublicKey
  location: package java.security.interfaces

This issue only arises when using Ed25519, such as when running the test suite. The JDK release flag can remain at JDK8, allowing the package to be used on JDK8 as long as Ed25519 is not required.

Fixes #34