Add Elxir 18 to the build matrix #38

Job	Run time
Check or calculate code quality metrics	26s
Build and test (1.12.3, 24.3)	4m 29s
Build and test (1.13.4, 24.3)	3m 48s
Build and test (1.13.4, 25.3)	2m 52s
Build and test (1.14.5, 24.3)	3m 36s
Build and test (1.14.5, 25.3)	2m 50s
Build and test (1.14.5, 26.2)	2m 49s
Build and test (1.15.8, 24.3)	3m 31s
Build and test (1.15.8, 25.3)	2m 49s
Build and test (1.15.8, 26.2)	2m 52s
Build and test (1.16.3, 24.3)	3m 36s
Build and test (1.16.3, 25.3)	2m 53s
Build and test (1.16.3, 26.2)	2m 57s
Build and test (1.17.3, 25.3)	3m 1s
Build and test (1.17.3, 26.2)	2m 52s
Build and test (1.17.3, 27.1)	2m 51s
Build and test (1.18.0, 27.1)	5m 21s
	53m 33s

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