chore(nimbus): remove GraphQL API (graphene-django) (#14892)

Because

* The Nimbus UI was fully migrated to Django form views (HTMX)
* Integration tests were decoupled from GraphQL in PRs #14815 and #14842
* The GraphQL API at `/api/v5/graphql` is no longer used by any client
* Removing it reduces maintenance burden, dependencies, and attack
surface

This commit

* Deletes the GraphQL schema, queries, mutations, types, and inputs
* Deletes the `GrapheneExceptionMiddleware`
* Removes `graphene-django` and `graphene-file-upload` from dependencies
* Removes `GRAPHENE` settings and `graphql_url` context processor
* Removes the `/api/v5/graphql` URL route (CSV, YAML, FML endpoints
preserved)
* Removes GQL-only serializers (`NimbusExperimentSerializer`,
`CloneSerializer`, and their mixins/validators) while keeping Review,
CSV, YAML, and FML serializers
* Removes unused `with_owner_features()` queryset method from models
* Deletes all graphene type stubs (`typings/graphene*`)
* Deletes GQL mutation, query, and serializer test files
* Adds test coverage for `TargetingMultipleKintoCollectionsError`
exception paths
* Net removal: ~12,800 lines across 107 files

Fixes #14890

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jaredlockhart

experimenter/experimenter/base/context_processors.py

@@ -1,6 +0,0 @@
-import graphene
-
-from experimenter.experiments.api.v5.mutations import Mutation
-from experimenter.experiments.api.v5.queries import Query
-
-schema = graphene.Schema(query=Query, mutation=Mutation)