RFR: JDK-8293768: Add links to JLS 19 and 20 from SourceVersion enum constants

[Joe Darcy]

Originally, I was only going to add a link to JLS 19 for when the docs for Java SE 19 went live, but at time of writing the JLS 19 link goes to the specifications landing page rather than giving a 404. Therefore, adding links to both JLS 19 and 20 know aren't dead links per se and will become better links as the JLS 19 and 20 pages are published in the future.

When the resolved JLS-version-specific links resolving the specification home page, the future I'll add the link to the specific JLS version when I add a new SourceVersion constant.

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

Commit messages:
 - JDK-8293768: Add links to JLS 19 and 20 from SourceVersion enum constants

Changes: https://git.openjdk.org/jdk/pull/10257/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10257&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293768
  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10257.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10257/head:pull/10257

PR: https://git.openjdk.org/jdk/pull/10257