Allowing templates with "." introduces some ambiguity. Is index.html.erb
a template named "index" with format "html", or is it a template named
"index.html" without a format? We know it's probably the former, but if
we asked ActionView to render "index.html" we would currently get some
combination of the two: a Template with index.html as the name and
virtual path, but with html as the format.

This deprecates having "." anywhere in the template's name, we should
reserve this character for specifying formats. I think in 99% of cases
this will be people specifying `index.html` instead of simply `index`.

This was actually once deprecated in the 3.x series (removed in
6c57177f) but I don't think we can rely
on nobody having introduced this in the past 8 years.