Require Go 1.19

This is what Debian 12 ships, so is probably a decent baseline for now.

Rewriting things for Go 1.18 is too much work.

.github/workflows/test.yml

@@ -5,7 +5,7 @@
     "test": {
       "strategy": {
         "matrix": {
-          "go-version": ["1.18.x", "1.21.x"],
+          "go-version": ["1.19.x", "1.22.x"],
           "os":         ["ubuntu-latest", "macos-latest", "windows-latest"]
         }
       },

README.md

@@ -27,7 +27,7 @@ should run in most environments. It's recommended you use a binary, or a tagged
 release if you build from source especially in CI environments. This prevents
 your tests from breaking on changes to tests in this tool.
 
-To compile from source you will need Go 1.18 or newer:
+To compile from source you will need Go 1.19 or newer:
 
     % go install github.com/toml-lang/toml-test/cmd/toml-test@latest
 

go.mod

@@ -1,6 +1,6 @@
 module github.com/toml-lang/toml-test
 
-go 1.18
+go 1.19
 
 require (
 	github.com/BurntSushi/toml v1.4.0