go.mod: bump minimum Go from 1.12 to Go 1.18

bradfitz · bradfitz · commit dc2076913ede · 2023-09-03T21:11:30.000-07:00
Go 1.18 is what's in Ubuntu 22.04 LTS (Jammy). The later Ubuntus have
1.19, 1.20, and 1.21 (the current Go release)

Go 1.18 seems fine. Go 1.18 came out 2022-03-15, about 1.5 years ago.
diff --git a/go.mod b/go.mod
@@ -1,3 +1,3 @@
 module github.com/bradfitz/gomemcache
 
-go 1.12
+go 1.18

Original file line number	Diff line number	Diff line change
`@@ -1,3 +1,3 @@`
`1`	`1`	`module github.com/bradfitz/gomemcache`
`2`	`2`
`3`		`-go 1.12`
	`3`	`+go 1.18`