RFR: 8348170: Unnecessary Hashtable usage in CSS.styleConstantToCssMap

[Andrey Turbanov]

There is a field `javax.swing.text.html.CSS#styleConstantToCssMap` which uses legacy `Hashtable` class.
As this map is read-only and all its content is initialized in `<clinit>` we can safely use immutable `Map` instead.

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Commit messages:
 - [PATCH] Unnecessary Hashtable usage in CSS.styleConstantToCssMap

Changes: https://git.openjdk.org/jdk/pull/23149/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23149&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348170
  Stats: 36 lines in 1 file changed: 2 ins; 0 del; 34 mod
  Patch: https://git.openjdk.org/jdk/pull/23149.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23149/head:pull/23149

PR: https://git.openjdk.org/jdk/pull/23149