remove support for 7.17.x

After internal discussion, the fix to the Fleet UI conflict
resolution has not been backported to 7.17, thus making this
upgrade a breaking change without a valid upgrade path (it would
still be possible to not upgrade policies and only create new ones
but this is uncharted territory).

Based on this we decided to remove support for 7.17 altogether
from gcp 2.0.0

packages/gcp/manifest.yml

@@ -15,7 +15,7 @@ categories:
   - google_cloud
   - cloud
 conditions:
-  kibana.version: ^7.17.0 || ^8.3.0
+  kibana.version: ^8.3.0
 screenshots:
   - src: /img/filebeat-gcp-audit.png
     title: filebeat gcp audit