KEP-3857: promote RecursiveReadOnlyMounts feature to beta

The gate has reached beta in v1.31.
refs:
- https://kep.k8s.io/3857
- k/k PR 125475

Fix issue 48614

Signed-off-by: Akihiro Suda <[email protected]>

Author: AkihiroSuda

content/en/docs/reference/command-line-tools-reference/feature-gates/recursive-read-only-mounts.md

@@ -9,6 +9,10 @@ stages:
   - stage: alpha
     defaultValue: false
     fromVersion: "1.30"
+    toVersion: "1.30"
+  - stage: beta
+    defaultValue: true
+    fromVersion: "1.31"
 ---
 Enables support for recursive read-only mounts.
 For more details, see [read-only mounts](/docs/concepts/storage/volumes/#read-only-mounts).