Switch to go-toml v2

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