Fix some default site redirect woes

[bep]

• The fix for Investigate the sites matrix file mount merge logic #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

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