chore(oidc): graduate webkey to stable (#10122)

# Which Problems Are Solved

Stabilize the usage of webkeys.

# How the Problems Are Solved

- Remove all legacy signing key code from the OIDC API
- Remove the webkey feature flag from proto
- Remove the webkey feature flag from console
- Cleanup documentation

# Additional Changes

- Resolved some canonical header linter errors in OIDC
- Use the constant for `projections.lock` in the saml package.

# Additional Context

- Closes #10029
- After #10105
- After #10061

internal/query/projection/instance_features.go

@@ -80,10 +80,6 @@ func (*instanceFeatureProjection) Reducers() []handler.AggregateReducer {
 				Event:  feature_v2.InstanceImprovedPerformanceEventType,
 				Reduce: reduceInstanceSetFeature[[]feature.ImprovedPerformanceType],
 			},
-			{
-				Event:  feature_v2.InstanceWebKeyEventType,
-				Reduce: reduceInstanceSetFeature[bool],
-			},
 			{
 				Event:  feature_v2.InstanceDebugOIDCParentErrorEventType,
 				Reduce: reduceInstanceSetFeature[bool],