RFR: 8297515: serialVersionUID fields are not annotated with @Serial

[Per Minborg]

This PR proposes adding `@Serial` annotations to certain fields participating in serialisation.

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Commit messages:
 - Annotate serailzation fields with @Serial

Changes: https://git.openjdk.org/jdk/pull/11347/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11347&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297515
  Stats: 27 lines in 7 files changed: 27 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11347.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11347/head:pull/11347

PR: https://git.openjdk.org/jdk/pull/11347