Note that this has only been a problem with "raw dates" in TOML files in /data and similar. The predefined front matter
dates `.Date` etc. are converted to a Go Time and has worked fine even after upgrading to v2 of the go-toml lib.
Fixes#9979
`go-toml/v2`'s unmarshaler does not specify zone name even if value has
offset explicitly.
To make time-formatting behaviour consistent, convert them into string
in hugo.
Close#8895