Remove Slack as a contact mechanism #3798

Job	Run time
git_checks	7s
format	32s
macos (macos11, serial)	4m 47s
windows (win2019, serial)	13m 56s
macos (macos12, serial)	5m 14s
docker (ubuntu-bionic)	14m 29s
windows (win2019, ompi)	19m 42s
CodeQL (cpp)	32m 38s
windows (win2022, serial)	13m 47s
linux_el8 (el8, icc, ompi)	59m 22s
windows (win2022, ompi)	21m 1s
linux_el8 (el8, oneapi, ompi)	55m 57s
linux_ubuntu (ubuntu20.04, gcc8, shared, ompi)	38m 12s
linux_ubuntu (ubuntu20.04, gcc9, shared, ompi)	34m 30s
linux_ubuntu (ubuntu20.04, gcc10, shared, ompi)	33m 17s
linux_ubuntu (ubuntu20.04, gcc11, shared, ompi)	37m 9s
linux_ubuntu (ubuntu20.04, clang6, shared, ompi)	27m 43s
linux_ubuntu (ubuntu20.04, clang10, shared, ompi)	33m 6s
linux_ubuntu (ubuntu20.04, gcc10, mpich)	57m 48s
linux_ubuntu (ubuntu20.04, gcc8, serial)	10m 16s
linux_ubuntu (ubuntu20.04, clang6, serial)	11m 10s
linux_ubuntu (ubuntu20.04, gcc8, static, ompi, build_only)	11m 8s
linux_ubuntu (ubuntu20.04, clang6, static, ompi, build_only)	9m 25s
linux_ubuntu (ubuntu20.04, gcc8, static, serial)	10m 28s
contract (lammps)	5m 53s
contract (tau)	23m 47s
build_and_test	1s
	9h 45m 25s

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