[jdk11u-dev] RFR: 8302694: [11u] Update GHA Boot JDK to 11.0.18

[Christoph Langer]

Bump boot JDK for Github Actions to 11.0.18

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Commit messages:
 - JDK-8302694

Changes: https://git.openjdk.org/jdk11u-dev/pull/1760/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1760&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302694
  Stats: 6 lines in 1 file changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/1760.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev pull/1760/head:pull/1760

PR: https://git.openjdk.org/jdk11u-dev/pull/1760