ci: Update actions/checkout from v4 to v6

[Copilot]

Bumps actions/checkout to v6 (latest major) in both CI workflow files, following the same pattern as git/git#2278.

Changed:

• .github/workflows/java-setup.yml: actions/checkout@v4 → actions/checkout@v6
• .github/workflows/datadog-synthetics.yml: actions/checkout@v4 → actions/checkout@v6

Checklist

• Use the owner account that submitted registrar_reserve to sign the project information, and fill in the signature.
• After each update of the project information, please sign the latest data and submit the signature.
• Add a description for your pr, the description includes the original data.
• Fill up the signature of the original data, signature account to Signature Auto check.

Signature Auto check

Para ID Owner & Signature Account:
<REPLACE THE OWNER>

Data that needs to be signed:
file

Signature Hash: <REPLACE THE SIGNATURE>