base-buster: Pin elasticsearch to 7.9.3

That should ease fixing the current swh-search build failure

[1] https://jenkins.softwareheritage.org/job/DSEA/job/tests/485/console

Related to T2791

base-buster/Dockerfile

@@ -25,7 +25,7 @@ RUN --mount=type=cache,id=apt-cache,target=/var/cache/apt --mount=type=cache,id=
   arcanist \
   build-essential \
   cassandra \
-  elasticsearch \
+  elasticsearch=7.9.3 \
   fuse3 \
   git-lfs \
   jq \

[Phabricator Migration user]

Some references in the commit message have been migrated:

• T2791 is now swh/devel/swh-search#2791 (closed)