Change maintainer label to opencontainers in Dockerfile (#40736)

The old way of specifying maintainers in a Dockerfile was to use the
MAINTAINER instruction. This is now deprecated and the maintainer label was
used instead.

Nowadays the recommended way to set a maintainer is through the
`org.opencontainers.image.authors` label.

See https://docs.docker.com/reference/build-checks/maintainer-deprecated/.

dev-tools/packaging/templates/docker/Dockerfile.tmpl

@@ -82,8 +82,8 @@ LABEL \
   org.opencontainers.image.licenses="{{ .License }}" \
   org.opencontainers.image.title="{{ .BeatName | title }}" \
   org.opencontainers.image.vendor="{{ .BeatVendor }}" \
+  org.opencontainers.image.authors="infra@elastic.co" \
   name="{{ .BeatName }}" \
-  maintainer="infra@elastic.co" \
   vendor="{{ .BeatVendor }}" \
   version="{{ beat_version }}{{if .Snapshot}}-SNAPSHOT{{end}}" \
   release="1" \