RFR: JDK-8293892: Add links to JVMS 19 and 20 from ClassFileFormatVersion enum constants

[Joe Darcy]

Analogous fix to one already done for javax.lang.model.SourceVersion (JDK-8293768), the links to JVMS 19 and 20 are not currently valid, but redirect to the spec language page

https://docs.oracle.com/javase/specs/index.html

The URLs will become valid when 19 and 20, respectively, ship.

-------------

Commit messages:
 - JDK-8293892: Add links to JVMS 19 and 20 from ClassFileFormatVersion enum constants

Changes: https://git.openjdk.org/jdk/pull/10298/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10298&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293892
  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10298.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10298/head:pull/10298

PR: https://git.openjdk.org/jdk/pull/10298