[jdk17] RFR: JDK-8268826: Cleanup Override in Context-Specific Deserialization Filters

[Roger Riggs]

Remove the unnecessary special case "OVERRIDE" in jdk.serialFilterFactory property.
Fix description in the example of a filter allowing platform classes.
Suppress some warnings about use of SecurityManager in tests.

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Commit messages:
 - JDK-8268827: Cleanup Implement Context-Specific Deserialization Filters

Changes: https://git.openjdk.java.net/jdk17/pull/85/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17&pr=85&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268826
  Stats: 29 lines in 4 files changed: 10 ins; 13 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk17/pull/85.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17 pull/85/head:pull/85

PR: https://git.openjdk.java.net/jdk17/pull/85