RFR: 8317264: In `Pattern.Bound`, make some constants `static final`

[Eamonn McManus]

It looks to have been an oversight that `final` was omitted. The fields are never assigned after initialization. `final` leads to shorter bytecode.

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Commit messages:
 - In `Pattern.Bound`, make some constants `static final`.

Changes: https://git.openjdk.org/jdk/pull/15967/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15967&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317264
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/15967.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15967/head:pull/15967

PR: https://git.openjdk.org/jdk/pull/15967