Bump minimum Go version to 1.18 (#232)

Author: int128

go.mod

@@ -1,6 +1,6 @@
 module github.com/int128/oauth2cli
 
-go 1.13
+go 1.18
 
 toolchain go1.23.5
 
@@ -11,3 +11,8 @@ require (
 	golang.org/x/oauth2 v0.25.0
 	golang.org/x/sync v0.10.0
 )
+
+require (
+	cloud.google.com/go/compute/metadata v0.3.0 // indirect
+	golang.org/x/sys v0.1.0 // indirect
+)

go.sum

@@ -1,6 +1,5 @@
 cloud.google.com/go/compute/metadata v0.3.0 h1:Tz+eQXMEqDIKRsmY3cHTL6FVaynIjX2QxYC4trgAKZc=
 cloud.google.com/go/compute/metadata v0.3.0/go.mod h1:zFmK7XCadkQkj6TtorcaGlCW1hT1fIilQDwofLpJ20k=
-github.com/google/go-cmp v0.5.9/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
 github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
 github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
 github.com/int128/listener v1.2.0 h1:Gj+wLX1mCfetZWJz0wi7343JuP8qGrYcbavNQR2xye4=