RFR: 8348648: Unnecessary Hashtable usage in javax.swing.text.html.CSS.LengthUnit

[Andrey Turbanov]

Two `Hashtable`'s in `javax.swing.text.html.CSS.LengthUnit` class are modified only within `<clinit>` block:
1. lengthMapping
2. w3cLengthMapping

We can replace them with immutable maps to avoid Hashtable synchronization overhead.

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

Changes: https://git.openjdk.org/jdk/pull/23198/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23198&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348648
  Stats: 21 lines in 1 file changed: 0 ins; 2 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/23198.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23198/head:pull/23198

PR: https://git.openjdk.org/jdk/pull/23198