Bumping CI to Test Against JDK 21

[jesswrd]

Bumping CI to Test Against JDK 21. This is part of the setup to use a robolectric version that supports SDK 36. This new robolectric version requires JDK 21 at minimum.

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩² ↩³

[gemini-code-assist]

Code Review

I've reviewed the changes to update the JDK version in the CI configuration. The changes are straightforward and align with the goal of preparing for a Robolectric upgrade. I have a couple of suggestions to improve build reproducibility by pinning the JDK to a more specific version, which is a common best practice and seems to be followed for other dependencies in this file. Please see my detailed comments below.

[gemini-code-assist]

For improved build reproducibility, it's a best practice to pin dependencies to a specific version. Other dependencies in this file, like curl (7.64.0), are pinned. To align with this practice and prevent potential build breakages, could we pin open_jdk to a more specific version (e.g., 21.0.2)? This would ensure build stability if a future JDK 21 update introduces incompatibilities.

          {"dependency": "open_jdk", "version": "version:21.0.2"},

[gemini-code-assist]

For consistency and build stability, let's also pin the open_jdk version here to a specific version (e.g., 21.0.2). This ensures that the Javadoc generation uses the same predictable JDK version as the tests.

          {"dependency": "open_jdk", "version": "version:21.0.2"}