refactor(elasticsearch): Keep the 9 variant beside the default image

Move elasticsearch-9/Dockerfile to elasticsearch/9.Dockerfile, the first
use of the layout added in 549cb9c. Both spellings name the same image,
so the published :elasticsearch-9 tag is unchanged and discovery still
reports the same 32 images.

The build context moves with the file, from elasticsearch-9/ to
elasticsearch/. Inert here -- both Dockerfiles are a bare FROM with no
COPY or ADD, so nothing resolves against the context at all.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: MinecraftFuns