Update OPNsense version requirement to 25.7.3 (#427)

Updates the minimum OPNsense version requirement for NetBird package availability from 26.0 (development version) to 25.7.3 (stable release), indicating the package is now available in a stable OPNsense release.

Author: bcmmbaga

src/pages/how-to/installation/opnsense.mdx

@@ -4,8 +4,7 @@ The NetBird client (agent) allows a peer to join a pre-existing NetBird deployme
 there are both managed and [self-hosted](https://docs.netbird.io/selfhosted/selfhosted-quickstart) options available.
 
 <Note>
-    The NetBird package is officially included starting from OPNsense `26.0`. At the time of writing, it is available via
-    the `opnsense-devel` firmware (`26.1.a_83`) and will be part of the next stable release.
+    The NetBird package is officially included starting from OPNsense `25.7.3`.
 </Note>