RFR: 8296900: CertificateValidity fields are not be optional

[Weijun Wang]

Make two fields inside `CertificateValidity` final.

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Commit messages:
 - the fix

Changes: https://git.openjdk.org/jdk/pull/11126/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11126&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296900
  Stats: 112 lines in 2 files changed: 71 ins; 37 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/11126.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11126/head:pull/11126

PR: https://git.openjdk.org/jdk/pull/11126