[Backport 5.3.9104] Redirect to sign-in provider when there is only one (#62007)

This PR makes the sign-in page always redirect if there is only a single sign-in provider, since there is nothing else to do on the sign in page otherwise.

## Test plan

Verified to work locally

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Backport 5653a5f from #61919

client/web/src/auth/SignInPage.tsx

@@ -86,8 +86,8 @@ export const SignInPage: React.FunctionComponent<React.PropsWithChildren<SignInP
     }
 
     const thirdPartyAuthProviders = nonBuiltinAuthProviders.filter(provider => shouldShowProvider(provider))
-    // If there is only one auth provider that is going to be displayed on dotcom, we want to redirect to it directly.
-    if (context.sourcegraphDotComMode && thirdPartyAuthProviders.length === 1) {
+    // If there is only one auth provider that is going to be displayed, we want to redirect to it directly.
+    if (thirdPartyAuthProviders.length === 1 && !builtInAuthProvider) {
         // Add '?returnTo=' + encodeURIComponent(returnTo) to thirdPartyAuthProviders[0].authenticationURL in a safe way.
         const redirectUrl = new URL(thirdPartyAuthProviders[0].authenticationURL, window.location.href)
         if (returnTo) {