Update releasePlan.cfg for July 2026 release

[adamfarley]

@copilot - Request for a PR that replaces the releasePlan.cfg values with these:

• jdk-26.0.2-dryrun-ga
• jdk-25.0.4-dryrun-ga
• jdk-21.0.12-dryrun-ga
• jdk-17.0.20-dryrun-ga
• jdk-11.0.32-dryrun-ga
• jdk-8u501-dryrun-ga