remove JSON file flag data source

[justindbaur]

🎟️ Tracking

bitwarden/server#7898

📔 Objective

Removes documentation for the flags.json file-based data source for local feature flag configuration, which is no longer supported after the server change linked above.

📸 Screenshots

N/A

[github-actions]

🤖 Bitwarden Claude Code Review

Overall Assessment: APPROVE

This documentation-only change removes the flags.json file-based data source from
docs/contributing/feature-flags.md, consistent with the linked server change
(bitwarden/server#7898) which confirms FlagDataFilePath is no longer supported. The flag source
table, the self-hosted caution admonition, and the surrounding prose were all updated together, so
no stale references to the file-based source remain in the page. I verified no other docs page links
to the removed Local configuration: JSON file section anchor, and that the remaining user secrets
example (globalSettings.launchDarkly.flagValues) is still a supported configuration path per the
server PR's backward-compatibility notes.

No findings.