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/api/grpc/feature/v2/converter.go

@@ -58,7 +58,6 @@ func instanceFeaturesToCommand(req *feature_pb.SetInstanceFeaturesRequest) (*com
 		UserSchema:                      req.UserSchema,
 		TokenExchange:                   req.OidcTokenExchange,
 		ImprovedPerformance:             improvedPerformanceListToDomain(req.ImprovedPerformance),
-		WebKey:                          req.WebKey,
 		DebugOIDCParentError:            req.DebugOidcParentError,
 		OIDCSingleV1SessionTermination:  req.OidcSingleV1SessionTermination,
 		DisableUserTokenEvent:           req.DisableUserTokenEvent,
@@ -77,7 +76,6 @@ func instanceFeaturesToPb(f *query.InstanceFeatures) *feature_pb.GetInstanceFeat
 		UserSchema:                          featureSourceToFlagPb(&f.UserSchema),
 		OidcTokenExchange:                   featureSourceToFlagPb(&f.TokenExchange),
 		ImprovedPerformance:                 featureSourceToImprovedPerformanceFlagPb(&f.ImprovedPerformance),
-		WebKey:                              featureSourceToFlagPb(&f.WebKey),
 		DebugOidcParentError:                featureSourceToFlagPb(&f.DebugOIDCParentError),
 		OidcSingleV1SessionTermination:      featureSourceToFlagPb(&f.OIDCSingleV1SessionTermination),
 		DisableUserTokenEvent:               featureSourceToFlagPb(&f.DisableUserTokenEvent),