build: set min Go version to 1.16 in go.mod

go.mod

@@ -17,7 +17,6 @@ require (
 	github.com/btcsuite/btcwallet/wtxmgr v1.3.1-0.20210822222949-9b5a201c344c
 	github.com/coreos/go-systemd v0.0.0-20190719114852-fd7a80b32e1f
 	github.com/davecgh/go-spew v1.1.1
-	github.com/fsnotify/fsnotify v1.4.9 // indirect
 	github.com/go-errors/errors v1.0.1
 	github.com/go-openapi/strfmt v0.19.5 // indirect
 	github.com/golang/protobuf v1.5.2
@@ -90,4 +89,4 @@ replace github.com/dgrijalva/jwt-go => github.com/golang-jwt/jwt v3.2.1+incompat
 
 // If you change this please also update .github/pull_request_template.md and
 // docs/INSTALL.md.
-go 1.15
+go 1.16