RFR: 8284026: Use unmodifiable collections where practical

[Pavel Rappo]

New code in jdk.javadoc tends to use `{List, Set, Map}.of` methods instead of similar methods from `Collections`, which are used in the existing jdk.javadoc code. Having a mix of these two families of methods is distracting. This PR upgrades existing code to `{List, Set, Map}.of` methods where practical.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/8046/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8046&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284026
  Stats: 74 lines in 22 files changed: 1 ins; 16 del; 57 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8046.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8046/head:pull/8046

PR: https://git.openjdk.java.net/jdk/pull/8046