add info about k8s-upgrade tests for release branches

Signed-off-by: Sunnatillo <[email protected]>

docs/e2e-test.md

@@ -106,12 +106,31 @@ in the ephemeral cluster either before pivoting or after re-pivoting.
 
 ### K8s upgrade tests
 
-Kubernetes version upgrade in target nodes. E2E has following k8s version
-upgrades:
+Kubernetes version upgrade in target nodes. We run three latest version
+upgrades in k8s-upgrade tests for main branch. And one kubernetes upgrade
+version for each release branch. When a new Kubernetes minor release is
+available, we will try to support it in an upcoming CAPM3 patch release
+(only in the latest supported CAPM3 minor release).
+
+For example:
+
+Main branch k8s-upgrade tests:
 
-- `v1.26` => `v1.27`
 - `v1.27` => `v1.28`
 - `v1.28` => `v1.29`
+- `v1.29` => `v1.30`
+
+Release 1.7 branch k8s-upgrade test:
+
+- `v1.29` => `v1.30`
+
+Release 1.6 branch k8s-upgrade test:
+
+- `v1.28` => `v1.29`
+
+Release 1.5 branch k8s-upgrade test:
+
+- `v1.26` => `v1.27`
 
 ### Live ISO test