fix(core): raise the rxfire floor to ^6.2.0 for its firebase 12 peers

[armando-navarro]

rxfire 6.2.0 shipped to npm latest on July 22, 2026 with firebase 12 in its peer range (FirebaseExtended/rxfire#152). This PR raises our dependency floor to require it.

Background

• rxfire 6.1.0 accepts firebase ^9 || ^10 || ^11 as peers. @angular/fire on main depends on firebase ^12.4.0, so a tree holding rxfire 6.1.0 gets a second firebase 11 installed next to firebase 12 (or, at best, an invalid-peer marker).
• Two installed SDK copies fail each other's instance checks at runtime; the first Firestore read throws Type does not match the expected instance. Did you pass a reference from a different Firestore SDK?.
• rxfire 6.2.0 widens the peer range to ^9 || ^10 || ^11 || ^12 and contains no other runtime change (the only other commit between 6.1.0 and 6.2.0 is CI-only).

What this changes

• src/package.json: rxfire ^6.1.0 to ^6.2.0 (the range the published package ships).
• root package.json: the same bump, and removes the overrides block that forced rxfire's firebase peer to ^12.4.0 in this repo's own install tree; rxfire 6.2.0 makes it unnecessary.
• package-lock.json: rxfire moves to 6.2.0; npm also synced the lockfile's stale root version 21.0.0-next.0 to 21.0.0-rc.0, matching package.json.

Why raise the floor when ^6.1.0 already resolves to 6.2.0

Fresh installs are unaffected: a clean project installing @angular/fire@21.0.0-rc.0 today already resolves rxfire 6.2.0 and a single firebase (verified). The floor is for upgrades: a lockfile that pins rxfire 6.1.0 satisfies ^6.1.0, survives an @angular/fire upgrade, and rxfire 6.1.0's peer set then re-creates the duplicate firebase 11. With ^6.2.0 the upgrade is forced onto the fixed release.

What this does not fix

An app whose own firebase dependency stays on 11.x still ends up with two SDK copies: rxfire binds the root firebase 11 while @angular/fire nests firebase 12. That is the mechanism behind #3684, #3681, and #3682, and the fix there is upgrading the app's firebase to ^12 together with @angular/fire; a schematics follow-up and the release notes will cover that path. Refs #3666.

Verification

• Clean-project install of 21.0.0-rc.0: resolves rxfire 6.2.0, one firebase@12.16.0 (rxfire's copy deduped), one @firebase/firestore.
• Repo tree after the change: rxfire 6.2.0, single firebase, no nested copy under rxfire.
• npm run build: the packed tarball's package.json carries rxfire: ^6.2.0.

[tyler-reitz]

Approve. Clean, minimal, well-justified; verified the rxfire 6.2.0 peer range and integrity hash against npm, and the before-state in package.json/src/package.json matches main.

Optional: since "app still pinned to firebase 11" is the more common user-facing symptom (#3684/#3681/#3682), consider linking the schematics follow-up issue in the PR body so reviewers can track that the full remediation lands.