RFR: 8345616: Unnecessary Hashtable usage in javax.swing.text.html.parser.Element

[Andrey Turbanov]

The Hashtable `javax.swing.text.html.parser.Element#contentTypes` is initialized only within `<clinit>` block.
We can replace it with immutable map to avoid Hashtable `synchronized` overhead.

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Commit messages:
 - [PATCH] Unnecessary Hashtable usage in javax.swing.text.html.parser.Element

Changes: https://git.openjdk.org/jdk/pull/21825/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21825&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345616
  Stats: 16 lines in 1 file changed: 0 ins; 5 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/21825.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21825/head:pull/21825

PR: https://git.openjdk.org/jdk/pull/21825