ci-signal: clarified timing for dropping n-3 release support in setup steps

Author: shecodesmagic

docs/release/role-handbooks/ci-signal/README.md

@@ -37,7 +37,8 @@ While we add test coverage for the new release branch we will also drop the test
     2. Modify the following at the `release-1.8` branch entry:
             * Change intervals (let's use the same as for `release-1.7`).
 2. Create a new dashboard for the new branch in: `test-infra/config/testgrids/kubernetes/sig-cluster-lifecycle/config.yaml` (`dashboard_groups` and `dashboards`).
-3. Remove old release branches and unused versions from the `cluster-api-prowjob-gen.yaml` file in [test-infra](https://github.com/kubernetes/test-infra/blob/master/config/jobs/kubernetes-sigs/cluster-api/) according to our policy documented in [Support and guarantees](../../../../CONTRIBUTING.md#support-and-guarantees). As we just added `release-1.8`, then we can now drop test coverage for the `release-1.5` branch.
+3. Do not remove old release branches during this setup phase.
+Per our policy documented in [Support and guarantees](../../../../CONTRIBUTING.md#support-and-guarantees), old release branches (e.g., `release-1.5`) should only be removed after the new release (v1.8.0) has been officially cut. This ensures continued support for n-3 versions throughout the current release cycle. Once `v1.8.0` is released, then test coverage for old release branches and unused versions (in this case `release-1.5`) may be removed from the `cluster-api-prowjob-gen.yaml` file in [test-infra](https://github.com/kubernetes/test-infra/blob/master/config/jobs/kubernetes-sigs/cluster-api/).
 4. Review the templates and modify/remove any outdated logic in the templates.  For example, when `release-1.7` is dropped from support, it can be removed from [this if statement](https://github.com/kubernetes/test-infra/blob/fa895d9f204e912e2bf0bd42221017a6dedf6065/config/jobs/kubernetes-sigs/cluster-api/templates/cluster-api-periodics-upgrades.yaml.tpl#L42).
 5. Regenerate the prowjob configuration running `make generate-test-infra-prowjobs` command from cluster-api repository. Before running this command, ensure to export the `TEST_INFRA_DIR` variable, specifying the location of the [test-infra](https://github.com/kubernetes/test-infra/) repository in your environment. For further information, refer to this [link](https://github.com/kubernetes-sigs/cluster-api/pull/9937).