fix: version selector shows v21 instead of next in dev mode

The version-manager.service was returning 'next' when VERSION.major
was '0' (development mode). Updated the patch to return 'v21' for
Angular.jp in dev mode.

Also updated versions.json to point v21 to angular.jp instead of v20.

Author: lacolaco

tools/adev-patches/replace-version-links.patch

@@ -1,18 +1,5 @@
-diff --git a/adev/src/assets/others/versions.json b/adev/src/assets/others/versions.json
-index 44dad0e1ef..610fdab042 100644
---- a/adev/src/assets/others/versions.json
-+++ b/adev/src/assets/others/versions.json
-@@ -5,7 +5,7 @@
-   },
-   {
-     "version": "v20",
--    "url": "https://v20.angular.dev/overview"
-+    "url": "https://angular.jp/overview"
-   },
-   {
-     "version": "v19",
 diff --git a/adev/src/app/core/services/version-manager.service.ts b/adev/src/app/core/services/version-manager.service.ts
-index d4e9fb86ea..eaf7969333 100644
+index bf0af31c22..56890e8586 100644
 --- a/adev/src/app/core/services/version-manager.service.ts
 +++ b/adev/src/app/core/services/version-manager.service.ts
 @@ -62,7 +62,7 @@ export class VersionManager {
@@ -24,3 +11,31 @@ index d4e9fb86ea..eaf7969333 100644
        transferCache: false,
        cache: 'no-cache',
      }),
+@@ -94,8 +94,12 @@ export class VersionManager {
+   );
+ 
+   readonly currentDocsVersion = computed(() => {
+-    // In devmode the version is 0, so we'll target next (which is first on the list)
+-    if (VERSION.major === '0' || VERSION.patch.includes('next')) {
++    // For Angular.jp, always target the stable version (v21)
++    if (VERSION.major === '0') {
++      return this.versions().find((v) => v.displayName === 'v21') ?? this.versions()[0];
++    }
++
++    if (VERSION.patch.includes('next')) {
+       return this.versions()[0];
+     }
+ 
+diff --git a/adev/src/assets/others/versions.json b/adev/src/assets/others/versions.json
+index 52ec12a661..07e1c78a5b 100644
+--- a/adev/src/assets/others/versions.json
++++ b/adev/src/assets/others/versions.json
+@@ -5,7 +5,7 @@
+   },
+   {
+     "version": "v21",
+-    "url": "https://v21.angular.dev/overview"
++    "url": "https://angular.jp/overview"
+   },
+   {
+     "version": "v20",