Update plugin test BK pipeline to parallel all steps

[andsel]

The Buildkite pipeline supported_plugins_test_pipeline make an explicit depenceny between Tier1 and Tier2, giving priority to the ffirst:

logstash/.buildkite/supported_plugins_test_pipeline.yml

Lines 8 to 34 in 3a753e4

	- group: "Tier1 plugins test group"
	key: "tier1_plugins"
	steps:
	- label: "Test Tier1 inputs"
	command: ./ci/test_supported_plugins.sh -t tier1 -k input
	- label: "Test Tier1 filters"
	command: ./ci/test_supported_plugins.sh -t tier1 -k filter
	- label: "Test Tier1 codecs"
	command: ./ci/test_supported_plugins.sh -t tier1 -k codec
	- label: "Test Tier1 outputs"
	command: ./ci/test_supported_plugins.sh -t tier1 -k output
	- label: "Test Tier1 integrations"
	command: ./ci/test_supported_plugins.sh -t tier1 -k integration
	- group: "Tier2 plugins test group"
	key: "tier2_plugins"
	# Wait all tier1 plugins testing ends before going on tier2
	depends_on: "tier1_plugins"
	steps:
	- label: "Test Tier2 inputs"
	command: ./ci/test_supported_plugins.sh -t tier2 -k input
	- label: "Test Tier2 filters"
	command: ./ci/test_supported_plugins.sh -t tier2 -k filter
	- label: "Test Tier2 codecs"
	command: ./ci/test_supported_plugins.sh -t tier2 -k codec
	- label: "Test Tier2 outputs"
	command: ./ci/test_supported_plugins.sh -t tier2 -k output

Remove this constraint, removing group definitions and flattening all the steps at the same level so that all can be launched in parallel.