v1_parser: use URI#open instead of URI.open (fluent#4854)

**Which issue(s) this PR fixes**:
Fixes #

**What this PR does / why we need it**:
By [CodeQL
documentation](https://codeql.github.com/codeql-query-help/ruby/rb-non-constant-kernel-open/),
it is safer to avoid using `URI.open`.

This is similar with fluent#4848

**Docs Changes**:

**Release Note**:

Signed-off-by: Shizuo Fujita <[email protected]>
Signed-off-by: Kentaro Hayashi <[email protected]>

Author: Watson1978

lib/fluent/config/v1_parser.rb

@@ -172,7 +172,7 @@ def eval_include(attrs, elems, uri)
           require 'open-uri'
           basepath = '/'
           fname = path
-          data = URI.open(uri) { |f| f.read }
+          data = u.open { |f| f.read }
           data.force_encoding('UTF-8')
           ss = StringScanner.new(data)
           V1Parser.new(ss, basepath, fname, @eval_context).parse_element(true, nil, attrs, elems)