[jdk21u-dev] RFR: 8331639: [21u]: Bump GHA bootstrap JDK to 21.0.3

[Christoph Langer]

>From time to time the GHA bootstrap JDK in the update releases should be bumped. So let's take the current verion 21.0.3 from Temurin.

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Commit messages:
 - JDK-8331639

Changes: https://git.openjdk.org/jdk21u-dev/pull/550/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=550&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331639
  Stats: 8 lines in 1 file changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/550.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/550/head:pull/550

PR: https://git.openjdk.org/jdk21u-dev/pull/550