Upgrade go-toml to v2

According to the official
[migration guide](https://github.com/pelletier/go-toml/tree/v2#migrating-from-v1),
there are no breaking changes that could affect this project.

Main differences are:

* `Default struct fields order`: Struct fields stop being alphabetically
  sorted, unless their sorting is changed in the struct definitions. Shouldn't be
  relevant.
* `No indentation by default`: Also not relevant for newly generated
  configuration files. Doesn't break reading existing configuration
  files.

go.mod

@@ -9,7 +9,7 @@ require (
 	github.com/dweymouth/go-mpv v0.0.0-20230406003141-7f1858e503ee
 	github.com/dweymouth/go-subsonic v0.0.0-20230513020020-0790f53c2868
 	github.com/google/uuid v1.3.0
-	github.com/pelletier/go-toml v1.9.3
+	github.com/pelletier/go-toml/v2 v2.0.8
 	github.com/zalando/go-keyring v0.2.1
 	golang.org/x/net v0.0.0-20220722155237-a158d28d115b
 )
@@ -35,7 +35,7 @@ require (
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/srwiley/oksvg v0.0.0-20220731023508-a61f04f16b76 // indirect
 	github.com/srwiley/rasterx v0.0.0-20210519020934-456a8d69b780 // indirect
-	github.com/stretchr/testify v1.8.0 // indirect
+	github.com/stretchr/testify v1.8.3 // indirect
 	github.com/tevino/abool v1.2.0 // indirect
 	github.com/yuin/goldmark v1.4.13 // indirect
 	golang.org/x/image v0.3.0 // indirect