Enable fastFallback by default (#7566)

okhttp-testing-support/src/main/kotlin/okhttp3/OkHttpClientTestRule.kt

@@ -115,7 +115,6 @@ class OkHttpClientTestRule : BeforeEachCallback, AfterEachCallback {
     var client = testClient
     if (client == null) {
       client = OkHttpClient.Builder()
-        .fastFallback(true) // Test this by default, since it'll soon be the default.
         .dns(SINGLE_INET_ADDRESS_DNS) // Prevent unexpected fallback addresses.
         .eventListenerFactory { ClientRuleEventListener(logger = ::addEvent) }
         .build()

okhttp/src/jvmMain/kotlin/okhttp3/OkHttpClient.kt

@@ -512,7 +512,7 @@ open class OkHttpClient internal constructor(
     internal val networkInterceptors: MutableList<Interceptor> = mutableListOf()
     internal var eventListenerFactory: EventListener.Factory = EventListener.NONE.asFactory()
     internal var retryOnConnectionFailure = true
-    internal var fastFallback = false
+    internal var fastFallback = true
     internal var authenticator: Authenticator = Authenticator.NONE
     internal var followRedirects = true
     internal var followSslRedirects = true
@@ -669,6 +669,8 @@ open class OkHttpClient internal constructor(
      * This implements Happy Eyeballs ([RFC 6555][rfc_6555]), balancing connect latency vs.
      * wasted resources.
      *
+     * Defaults to enabled, call with [fastFallback] = false to revert to 4.x behaviour.
+     *
      * [rfc_6555]: https://datatracker.ietf.org/doc/html/rfc6555
      */
     fun fastFallback(fastFallback: Boolean) = apply {