Update CODEOWNERS: rename team-fm-next to team-fm-foundations

[kparkinson-ld]

Summary

Renames the CODEOWNERS team reference from @launchdarkly/team-fm-next to @launchdarkly/team-fm-foundations to reflect the team's new name. This is part of a batch update across multiple repos in the org.

Review & Testing Checklist for Human

• Verify that the GitHub team @launchdarkly/team-fm-foundations exists and has the expected members
• Confirm no other references to team-fm-next remain in this repo outside of CODEOWNERS (e.g., CI configs, docs)

Notes

• This repo's CODEOWNERS only had a single reference to the old team name.
• Part of an org-wide rename affecting multiple repositories.

Link to Devin session: https://app.devin.ai/sessions/8abe80d07e414755864c1b9c67215686
Requested by: @kparkinson-ld

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