change: use apisix.enable_http2 to enable HTTP/2 in APISIX #16

Job	Run time
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/http3/admin)	10m 43s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[a-k]*)	29m 20s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[l-z]*)	30m 11s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/admin t/cli t/config-center-yaml...	22m 25s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/node t/pubsub t/router t/script ...	22m 36s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/http3/admin)	11m 15s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[a-k]*)	29m 59s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[l-z]*)	29m 19s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/admin t/cli t/config-center-yaml t/cont...	21m 34s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/node t/pubsub t/router t/script t/secre...	22m 34s
	3h 49m 56s

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