Update isConcurrent RTR option usage (#28546)

Reverting some of #27804 which
renamed this option to stable. This PR just replaces internal usage to
make upcoming PRs cleaner.

Keeping isConcurrent unstable for the next major release in order to
enable a broader deprecation of RTR and be consistent with concurrent
rendering everywhere for next major.
(#28498)
- Next major will use concurrent root
- The old behavior (legacy root by default, concurrent root with
unstable option) will be preserved for React Native until new
architecture is fully shipped.
- Flag and legacy root usage can be removed after RN dependency is
unblocked without an additional breaking change

DiffTrain build for [3832730](3832730)

compiled/facebook-www/REVISION

@@ -1 +1 @@
-b09e102ff1e2aaaf5eb6585b04609ac7ff54a5c8
+38327309a4172a52cf94e8e264990bedafc1df6e

compiled/facebook-www/ReactTestRenderer-dev.classic.js

@@ -26055,7 +26055,7 @@ if (__DEV__) {
       return root;
     }
 
-    var ReactVersion = "18.3.0-www-classic-91cd5cc2";
+    var ReactVersion = "18.3.0-www-classic-2256e5ca";
 
     // Might add PROFILE later.
 
@@ -26959,10 +26959,7 @@ if (__DEV__) {
           createNodeMock = options.createNodeMock;
         }
 
-        if (
-          options.unstable_isConcurrent === true ||
-          options.isConcurrent === true
-        ) {
+        if (options.unstable_isConcurrent === true) {
           isConcurrent = true;
         }
 

compiled/facebook-www/ReactTestRenderer-dev.modern.js

@@ -26055,7 +26055,7 @@ if (__DEV__) {
       return root;
     }
 
-    var ReactVersion = "18.3.0-www-modern-d806e7dc";
+    var ReactVersion = "18.3.0-www-modern-e9ba68a0";
 
     // Might add PROFILE later.
 
@@ -26959,10 +26959,7 @@ if (__DEV__) {
           createNodeMock = options.createNodeMock;
         }
 
-        if (
-          options.unstable_isConcurrent === true ||
-          options.isConcurrent === true
-        ) {
+        if (options.unstable_isConcurrent === true) {
           isConcurrent = true;
         }