[ruby/net-http] Drop support for Ruby 2.6 #207

Job	Run time
make (check, --zjit-disable-hir-opt --zjit-call-threshold=1, -T --zjit-disable-hir-opt -T --zjit-...	26m 8s
make (check, --zjit-call-threshold=1, -T --zjit-call-threshold=1, --enable-zjit=dev)	17m 44s
make (ruby, combo build test, --enable-yjit --enable-zjit)	4m 3s
make (zjit-check, --enable-yjit=dev --enable-zjit, 1.85.0)	4m 34s
ruby-bench (--zjit-call-threshold=2, --warmup=1 --bench=1, --enable-zjit=dev_nodebug)	14m 36s
ZJIT macOS result	2s
	1h 7m 7s

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