ci: add memcached 1.6.22 to the test matrix #296

Job	Run time
debug coverage (macos-12, clang)	7m 30s
debug memcached (master)	3m 49s
debug memcached (1.6.22)	4m 16s
debug sanitizer (address;undefined)	4m 10s
debug memcached (1.6.17)	3m 26s
debug sanitizer (thread)	3m 32s
debug memcached (1.6.9)	3m 39s
debug memcached (1.5.22)	2m 51s
debug memcached (1.5.6)	3m 38s
debug coverage (ubuntu-22.04, gnu)	3m 38s
release (macos-12)	1m 34s
release (macos-11)	1m 57s
release (macos-10.15)	1s
release (ubuntu-22.04, gnu, new)	1m 53s
release (ubuntu-22.04, gnu, cur)	1m 47s
release (ubuntu-22.04, gnu, old)	1m 43s
release (ubuntu-22.04, clang, new)	1m 38s
release (windows-2022, msvc, cur)	4m 26s
release (ubuntu-22.04, clang, cur)	1m 27s
release (windows-2022, mingw, cur)	4m 50s
release (ubuntu-22.04, clang, old)	1m 4s
release (windows-2019, msvc, cur)	4m 25s
release (ubuntu-20.04, gnu, new)	1m 13s
release (windows-2019, mingw, cur)	5m 1s
release (ubuntu-20.04, gnu, cur)	1m 19s
release (ubuntu-20.04, gnu, old)	1m 13s
release (ubuntu-20.04, clang, new)	1m 32s
release (ubuntu-20.04, clang, cur)	1m 30s
release (ubuntu-20.04, clang, old)	1m 37s
	1h 20m 39s

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