chore(.github): test scylla 2026.3 instead of scylla 2025.4

Scylla 2025.4 is no longer supported:
https://docs.scylladb.com/stable/versioning/version-support.html
Scylla 2026.3 has its first rc image.
Because of that, scylla 2025.4 is removed and scylla 2026.3 is
added to SM CI test matrix.

Author: Michal-Leszczynski

.github/cfg/integration-test-cfg.yaml

@@ -8,21 +8,6 @@
   tablets: enabled
   ssl-enabled: false
 
-- scylla-version: scylla:2025.4.10
-  ip-family: IPV4
-  tablets: disabled
-  ssl-enabled: true
-  backup-method: native
-  restore-method: rclone
-
-- scylla-version: scylla:2025.4.10
-  ip-family: IPV6
-  tablets: enabled
-  ssl-enabled: false
-  backup-method: rclone
-  restore-method: rclone
-  backup-provider: gcs
-
 - scylla-version: scylla:2026.1.7
   ip-family: IPV4
   tablets: enabled
@@ -52,6 +37,21 @@
   backup-method: rclone
   restore-method: rclone
 
+- scylla-version: scylla:2026.3.0-rc1
+  ip-family: IPV4
+  tablets: disabled
+  ssl-enabled: true
+  backup-method: native
+  restore-method: rclone
+
+- scylla-version: scylla:2026.3.0-rc1
+  ip-family: IPV6
+  tablets: enabled
+  ssl-enabled: false
+  backup-method: rclone
+  restore-method: rclone
+  backup-provider: gcs
+
 - scylla-version: scylla-nightly:latest
   ip-family: IPV4
   tablets: disabled