update readme Proxmox compatibility

due to proxmox API changes the foreman plugin is currently not compatible with the recent proxmox version

README.md

@@ -34,7 +34,7 @@ You can support the plugin development via the following methods:
 |>=0.12 |>=6.1|>=0.11|>=2.0|>=2.5|
 |>=0.14 |>=6.2|=0.13.0|>=2.4|>=2.7|
 |>=0.14 |>=6.2|>=0.13.1|>=2.3|>=2.5|
-|>=0.14 |>=6.2|>=0.14.0|>=2.5|>=2.5|
+|>=0.14 |>=6.2,<=8.0|>=0.14.0|>=2.5|>=2.5|
 
 ## Installation