ci: Update Go versions (Go 1.16 required)

We now use the io/fs package.

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       matrix:
         os: [ ubuntu-latest, macos-latest, windows-latest ]
-        go: [ '1.14', '1.15' ]
+        go: [ '1.16', '1.17' ]
 
     runs-on: ${{ matrix.os }}
 

filestorage.go

@@ -64,6 +64,8 @@ import (
 // that is probably less severe a consequence than infinite loops.
 //
 // See https://github.com/caddyserver/caddy/issues/4448 for discussion.
+// See commit 468bfd25e452196b140148928cdd1f1a2285ae4b for where we
+// switched away from using .unlock files.
 type FileStorage struct {
 	Path string
 }