chore: remove internal Node.js 18 build details (#1449)

Author: MikeMcC399

base-internal/README.md

@@ -20,3 +20,11 @@ for example:
 - `cypress/base-internal:ubuntu24-node18`
 
 Images which do not contain an operating system in their tag are based on Debian.
+
+## Obsolete Releases
+
+Build information for the following internal images is no longer kept in the default branch of the repo and is instead linked to a historical commit where the information is preserved:
+
+- [base-internal/releases/node-18](https://github.com/cypress-io/cypress-docker-images/tree/d7236afec49a2fb9be8f81c8d34f20e6166bcc57/base-internal/releases/node-18)
+
+The related internal images remain in the [cypress/base-internal](https://hub.docker.com/r/cypress/base-internal/tags) Docker Hub container image repository.