Backporting to ESR branches hits a flowzone error as both pull_request
and pull_request_target run and this only works for master.
Removing the pull_request_target run for ESR branches fixes this, but
also removes the possibility of external pull requests into ESR branches,
which we don't actually need.
[skip ci]
Changelog-entry: Remove pull_request_target event run for ESR targets
Signed-off-by: Alex Gonzalez <alexg@balena.io>
