Update CODEOWNERS: rename team-fm-next to team-fm-foundations (#58)

## Summary

Replaces `@launchdarkly/team-fm-next` with
`@launchdarkly/team-fm-foundations` in the CODEOWNERS file to reflect
the team rename.

This is one of several PRs making this change across the launchdarkly
org.

## Review & Testing Checklist for Human

- [ ] Verify that `@launchdarkly/team-fm-foundations` is a valid GitHub
team in the launchdarkly org
- [ ] Confirm no other references to `team-fm-next` remain in this repo

### Notes

Repos updated as part of this effort: `ld-find-code-refs`,
`find-code-references-in-pull-request`, `find-code-references`,
`ld-vscode`, `ld-openapi`. The `agent-skills` repo already had the
correct team name.

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

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Author: kparkinson-ld

CODEOWNERS

@@ -1 +1 @@
-* @launchdarkly/team-fm-next
+* @launchdarkly/team-fm-foundations