docs: drop removed token-exchange mode; deprecate superseded auth ADRs

The OAuth token-exchange deployment mode was removed (ADR-022) and has no
implementation — only a vestigial `enable_token_exchange` flag remains. Its
documentation still presented it as a usable mode, which misleads self-hosters.
The only supported deployment modes are single_user_basic, multi_user_basic,
and login_flow.

Token-exchange removals (how-to/config for a removed mode):
- delete docs/CRITICAL-TOKEN-EXCHANGE-PATTERN.md
- delete docs/oauth-architecture-comparison.md (orphaned; labelled the removed
  pass-through mode as "current implementation")
- env.sample: drop the "OAUTH TOKEN EXCHANGE MODE" section
- docker-compose.yml: drop ENABLE_TOKEN_EXCHANGE/TOKEN_EXCHANGE_CACHE_TTL from
  the keycloak service (dead flags)
- docs/webhook-management-guide.md: drop the token-exchange deployment section
- docs/configuration-migration-v2.md: drop the token-exchange migration scenario
- docs/observability.md: drop the never-emitted mcp_oauth_token_exchange_total

Auth ADR status corrections:
- ADR-004: Draft -> Superseded by ADR-022/ADR-023 (token-exchange/federated
  design not adopted); note the three supported modes.
- ADR-002: extend the deprecation pointer to ADR-022/ADR-023.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/observability.md

@@ -72,7 +72,6 @@ The Helm chart has moved to a [separate repository](https://github.com/cbcoutinh
 ### OAuth Flow Metrics
 
 - `mcp_oauth_token_validations_total` - Token validation count
-- `mcp_oauth_token_exchange_total` - Token exchange operations
 - `mcp_oauth_token_cache_hits_total` - Cache hit/miss rate
 - `mcp_oauth_refresh_token_operations_total` - Refresh token storage ops