Use --qualifier in release manager (elastic#16907)

This commit uses the new --qualifier parameter in the release manager
for publishing dra artifacts. Additionally, simplifies the expected
variables to rely on a simple `VERSION_QUALIFIER`.

Snapshot builds are skipped when VERSION_QUALIFIER is set.
Finally, for helping to test DRA PRs, we also allow passing the `DRA_BRANCH`  option/env var
to override BUILDKITE_BRANCH.

Closes elastic/ingest-dev#4856

.buildkite/scripts/dra/common.sh

@@ -25,7 +25,7 @@ function save_docker_tarballs {
 # Since we are using the system jruby, we need to make sure our jvm process
 # uses at least 1g of memory, If we don't do this we can get OOM issues when
 # installing gems. See https://github.com/elastic/logstash/issues/5179
-export JRUBY_OPTS="-J-Xmx4g"
+export JRUBY_OPTS="-J-Xmx2g"
 
 # Extract the version number from the version.yml file
 # e.g.: 8.6.0