Fix some default site redirect woes

Fix some default site redirect woes

* The fix for #14357 yesterday sadly had the assumption that default language/version/role always was the first site in the sites matrix. This is common, but not always true.
* Also, that fix forgot to add a redirect the other way when `defaultContentLanguageInSubdir` was disabled, e.g. from `/en/` to `/`.
* This commit also renames config option `DisableDefaultDimensionRedirect` to `DisableDefaultSiteRedirect`. This is stricly a breaking change, but it's only been out for a day, and the old name didn't make much sense.

Fixes #14361