CLD-5783 Implement rolling upgrade tests (#27842)

* Make server.prepare idempotent, don't cleanup containers on server.start
* Implement rolling upgrade E2E tests
* Apply Akis' suggestions from code review
* Apply Saturn's suggestions from review
* Communicate RollingRelease on GHA and test automation results channel
* Implement E2E test case versioning for rolling release tests

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Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: Akis Maziotis <akis.maziotis@mattermost.com>

e2e-tests/.ci/server.cloud_init.sh

@@ -2,10 +2,14 @@
 set -e -u -o pipefail
 cd "$(dirname "$0")"
 . .e2erc
+[ -f .env.cloud ] && . .env.cloud
 
 if [ "$SERVER" != "cloud" ]; then
   mme2e_log "Skipping cloud instance initialization: operation supported only for cloud server, but running with SERVER='$SERVER'"
   exit 0
+elif [ -n "${MM_CUSTOMER_ID:-}" ]; then
+  mme2e_log "Skipping cloud user creation: customer with ID '$MM_CUSTOMER_ID' is already configured. Please run 'make cloud-teardown' before creating a new user."
+  exit 0
 fi
 
 mme2e_log "Initializing cloud tests"