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/oidc/auth_request_converter.go

@@ -140,13 +140,8 @@ func HttpHeadersFromContext(ctx context.Context) (userAgent, acceptLang string)
 	if !ok {
 		return
 	}
-	if agents, ok := ctxHeaders[http_utils.UserAgentHeader]; ok {
-		userAgent = agents[0]
-	}
-	if langs, ok := ctxHeaders[http_utils.AcceptLanguage]; ok {
-		acceptLang = langs[0]
-	}
-	return userAgent, acceptLang
+	return ctxHeaders.Get(http_utils.UserAgentHeader),
+		ctxHeaders.Get(http_utils.AcceptLanguage)
 }
 
 func IpFromContext(ctx context.Context) net.IP {