[chore]: support only the last 3 versions of go (#1416)

The go language only provides support for the two versions before the
latest one, essentially supporting 3 versions.

I want to try moving to a world in which we do the same, with best
effort on any versions beyond the past 2. It doesn't mean it won't work,
but I'd prefer to keep things as simple as I can with the smallest
possible area for support and maintenance.

Author: nlopes

.github/workflows/test.yml

@@ -12,10 +12,6 @@ jobs:
     strategy:
       matrix:
         go:
-          - "1.18"
-          - "1.19"
-          - "1.20"
-          - "1.21"
           - "1.22"
           - "1.23"
           - "1.24"
@@ -33,7 +29,7 @@ jobs:
     steps:
       - uses: actions/setup-go@v5
         with:
-          go-version: "1.20"
+          go-version: "1.22"
           cache: false
       - uses: actions/checkout@v4
         with:

go.mod

@@ -1,11 +1,14 @@
 module github.com/slack-go/slack
 
-go 1.16
+go 1.22
 
 require (
-	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/go-test/deep v1.0.4
 	github.com/gorilla/websocket v1.4.2
-	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/stretchr/testify v1.2.2
 )
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+)