Deprecate the spaceless filter

[fabpot]

Closes #3576

Deprecate the spaceless filter for the following reasons:

• The performance is bad (as the work is done at runtime via a regexp)
• Optimizing the size of an HTML doc server side is "almost never" a good idea (compression is better and enough)
• There are some edge cases where you want to keep some spaces (see [BUG] Extension::spaceless: Algorithm removes spaces in the text and causes the meaning to be broken. #3576)
• Controlling whitespace is possible and fine-grained via the dedicated Twig modifiers on {{ }}

If someone find it useful, re-creating it is trivial (return trim(preg_replace('/>\s+</', '><', $content ?? ''));),
but with so many caveats and not so many use cases, I think it does not belong to core.

[stof]

This filter should be marked as deprecated in the filter definition, so that using the filter reports the deprecation at compile time (and so that the linter also reports it)