Tests: fixed proxy_cache_age.t with older Perl versions. The test used regular expressions with "{,n}" quantifier with empty lower bound, which is only available starting with Perl 5.34.0. Further, it used variable-length look-behind assertions, which are only available starting with Perl 5.30.0 and emit experimental warning till Perl 5.36.0. Fix is to rewrite regular expressions in question using "(?>pattern)" instead (an independent subexpression), which is available since at least Perl 5.005.