[FR] Remove Endgame support after EOL

### Repository Feature

Core Repo (rule management, validation, testing, lib, cicd, etc.)

### Problem Description

Elastic Endgame is EOL. This repo still treats it as a first-class data source: many rules list `endgame-*` in `index`, tag integrations with "Data Source: Elastic Endgame", and Python validation pulls Endgame stack schemas (`detection_rules/endgame`, `rule.py`, `integrations.py`, tests, version lock metadata). That no longer matches supported deployments and adds maintenance cost for no gain.

### Desired Solution

Strip Endgame end-to-end in one coordinated effort:

- Rules and building blocks: remove `endgame-*` from `index` arrays and drop Endgame from `integration` / related metadata wherever it appears. Re-run validation so remaining indices stay consistent.
- Library and tooling: remove or gate Endgame schema loading, stack version mappings, and any CLI or test paths that assume `endgame-*`. Clean docs and `pyproject.toml` keywords if they still call out Endgame.
- Confirm nothing in CI, hunting queries, or NDJSON exports still references Endgame-only fields or indices.

Order of work can follow whatever minimizes breakage for open PRs (big mechanical PR vs staged batches).

### Considered Alternatives

Leave rules as-is for historical installs only. Rejected for this tracker because EOL means we should not imply ongoing support in prebuilt packages.

### Additional Context

Initial grep shows widespread `endgame-*` / Endgame integration strings across `rules/`, `rules_building_block/`, and core Python under `detection_rules/`. Use this issue as the umbrella; link child PRs here.