[version] introduce `OT_THREAD_VERSION_1_4` #1400

Job	Run time
Examples	2m 38s
Stress network-forming	21m 44s
Stress commissioning	17m 5s
Stress connectivity	16m 35s
Stress network-latency	2m 41s
Stress multicast-performance	2m 26s
Stress otns-performance	2m 27s
Unittests	3m 9s
upload-coverage	1s
	1h 8m 46s

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