Switch to go-toml v2

We have been using `go-toml` for language files only. This commit makes it the only TOML library.

It's spec compliant and very fast.

A benchark building a site with 200 pages with TOML front matter:

```bash
name                                  old time/op    new time/op    delta
SiteNew/Regular_TOML_front_matter-16    48.5ms ± 1%    47.1ms ± 1%  -2.85%  (p=0.029 n=4+4)

name                                  old alloc/op   new alloc/op   delta
SiteNew/Regular_TOML_front_matter-16    16.9MB ± 0%    16.7MB ± 0%  -1.56%  (p=0.029 n=4+4)

name                                  old allocs/op  new allocs/op  delta
SiteNew/Regular_TOML_front_matter-16      302k ± 0%      296k ± 0%  -2.20%  (p=0.029 n=4+4)
```

Note that the front matter unmarshaling is only a small part of building a site, so the above is very good.

Fixes #8801

parser/frontmatter.go

@@ -20,7 +20,7 @@ import (
 
 	"github.com/gohugoio/hugo/parser/metadecoders"
 
-	"github.com/BurntSushi/toml"
+	toml "github.com/pelletier/go-toml/v2"
 
 	yaml "gopkg.in/yaml.v2"
 )

parser/frontmatter_test.go

@@ -31,9 +31,9 @@ func TestInterfaceToConfig(t *testing.T) {
 		// TOML
 		{map[string]interface{}{}, metadecoders.TOML, nil, false},
 		{
-			map[string]interface{}{"title": "test 1"},
+			map[string]interface{}{"title": "test' 1"},
 			metadecoders.TOML,
-			[]byte("title = \"test 1\"\n"),
+			[]byte("title = \"test' 1\"\n"),
 			false,
 		},
 

parser/metadecoders/decoder.go

@@ -24,7 +24,7 @@ import (
 	"github.com/gohugoio/hugo/common/herrors"
 	"github.com/niklasfasching/go-org/org"
 
-	"github.com/BurntSushi/toml"
+	toml "github.com/pelletier/go-toml/v2"
 	"github.com/pkg/errors"
 	"github.com/spf13/afero"
 	"github.com/spf13/cast"