Update Go versions we test against, update dependencies.

Author: Aaron Hopkins

.github/workflows/go-test.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        go: ['1.13', '1.14', '1.15', '1.16']
+        go: ['1.15', '1.16', '1.17', '1.18']
 
     steps:
       - uses: actions/setup-go@v2

go.mod

@@ -1,3 +1,3 @@
 module github.com/die-net/http-tarpit
 
-go 1.16
+go 1.15