Update doc to new UI (#1993)

This pull request includes a change to the
`docs/quickstart/installation.mdx` file in the `odigos ui`
documentation. The change updates the instructions for accessing the
legacy UI instead of the experimental new UI.

Documentation update:

*
[`docs/quickstart/installation.mdx`](diffhunk://#diff-4f768f9645352fa629b2c955f5d407945d5000f12746fb50929cc70b599350f2L79-R87):
Changed the section from "Experimental: New UI with --beta flag" to
"Using the Legacy UI" and updated the corresponding command and warning
message.

docs/quickstart/installation.mdx

@@ -76,14 +76,13 @@ odigos ui
 
 By default, Odigos UI is available at http://localhost:3000.
 
-## Experimental: New UI with --beta flag
+## Using the Legacy UI
 
 ```bash
-odigos ui --beta
+odigos ui --legacy
 ```
 
 <Warning>
-  The --beta flag provides access to a new, improved UI. Keep in mind that this
-  is an experimental feature and may include changes or incomplete
-  functionality.
+  The new UI is the default experience. The legacy UI remains available for
+  users who prefer the previous version.
 </Warning>