ci: point the @since reconciliation at the real module source roots (#258) (CP: 25.3)

[vaadin-bot]

This PR cherry-picks changes from the original PR #258 to branch 25.3.

Original PR description

Summary

The "Update @SInCE tags" workflow ran but never changed anything, because it looked for Java sources in directories that do not exist in this repo. This fixes the paths so the reconciliation actually finds our code.

What changed

CI only — no product code or public API is touched, so nothing changes for users of the library.

• The reconcile-since-tags action defaults each artifact's source root to <artifactId>/src/main/java. Our module directories are named after the module (shared, junit6, spring, quarkus), not after the published artifact, so every source root was missing and all four modules were skipped. The workflow finished green in under a minute with an empty diff and no PR.
• Source roots are now spelled out with the action's <artifact>=<source-root> syntax. A local run reconciles 39 files.
• Bumped the pinned action version. The new version fails the run when a configured source root does not exist, instead of skipping it quietly, so this kind of misconfiguration cannot pass unnoticed again. It also refuses to build an index from a failed release download.
• Added .since-index (the action's local cache directory) to .gitignore.