Switch io_service -> io_context #1267

Job	Run time
lint	1m 15s
coverage	4m 30s
all-builds	15m 29s
test-cxx (ubuntu-20.04, g++-9)	2m 10s
test-cxx (ubuntu-22.04, g++-12)	2m 19s
test-python (ubuntu-20.04, gcc-9, g++-9, 3.9)	2m 28s
test-cxx (ubuntu-20.04, clang++-10)	2m 17s
test-cxx (ubuntu-22.04, clang++-14)	1m 59s
test-python (ubuntu-22.04, gcc-12, g++-12, 3.12)	2m 11s
test-cxx (macos-14, clang++)	1m 6s
test-python (ubuntu-20.04, clang-10, clang++-10, 3.12)	2m 10s
test-python (ubuntu-22.04, clang-14, clang++-14, 3.12)	2m 2s
test-python (macos-14, clang, clang++, 3.12)	1m 11s
cibuildwheel-macos	0s
sdist	0s
cibuildwheel-linux	0s
combine	0s
	41m 7s

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