Docs: Update version references to v0.21.1 in READMEs and docs (#10761)

### What problem does this PR solve?

- Update version tags in README files (including translations) from
v0.21.0 to v0.21.1
- Modify Docker image references and documentation to reflect new
version
- Update version badges and image descriptions
- Maintain consistency across all language variants of README files

### Type of change

- [x] Documentation Update

admin/client/README.md

@@ -48,7 +48,7 @@ It consists of a server-side Service and a command-line client (CLI), both imple
 1.  Ensure the Admin Service is running.
 2.  Install ragflow-cli.
     ```bash
-    pip install ragflow-cli==0.21.0
+    pip install ragflow-cli==0.21.1
     ```
 3.  Launch the CLI client:
     ```bash

admin/client/admin_client.py

@@ -370,7 +370,7 @@ class AdminCLI(Cmd):
                         self.session.headers.update({
                             'Content-Type': 'application/json',
                             'Authorization': response.headers['Authorization'],
-                            'User-Agent': 'RAGFlow-CLI/0.21.0'
+                            'User-Agent': 'RAGFlow-CLI/0.21.1'
                         })
                         print("Authentication successful.")
                         return True