Only parse target Ruby from gemspec if array elements are strings

This fixes an issue mentioned in [this
comment](#12579 (comment))
on issue #12579. (I believe that the main/original issue mentioned in
issue #12579 has already been fixed by #12732.)

This change is a related/similar followup to #12732 (5026393). That PR
avoided an exception that was occurring in the
`RuboCop::TargetRuby::GemspecFile` target Ruby finder when a project's
gemspec determines the value of `required_ruby_version` in some dynamic
way (such as by reading from a `.ruby-version` file).

However, #12732 did not handle a case where the dynamic value for the
gemspec's `required_ruby_version` is wrapped within an array, and, in
such a case, currently, an exception like the following will occur:

```
Gem::Requirement::BadRequirementError:
  Illformed requirement [">= \#{File.read('.ruby-version').rstrip}"]
  ./lib/rubocop/target_ruby.rb:123:in `new'
  ./lib/rubocop/target_ruby.rb:123:in `find_default_minimal_known_ruby'
  ./lib/rubocop/target_ruby.rb:83:in `find_version'
  ./lib/rubocop/target_ruby.rb:29:in `initialize'
  ./lib/rubocop/target_ruby.rb:259:in `new'
```

(As with the scenario that _was_ fixed by #12732, I believe that this is
essentially a "latent" issue that is now occurring in more projects than
previously, due to #12645 having increased the precedence of the
`GemspecFile` target Ruby finder in the `RuboCop::TargetRuby::SOURCES`
list.)

This change avoids this exception by only attempting to parse a target
Ruby version from an array value for a gemspec's `required_ruby_version`
if every value in the array is a string. When the values within the
`required_ruby_version` array are _not_ all strings, now, rather than
raising an unhandled exception, the `GemspecFile` finder will return
`nil` for the target Ruby version, and so we will continue on to the
other TargetRuby finder classes, in order to find a target Ruby version.

changelog/fix_only_parse_target_ruby_from_gemspec_if_array.md

@@ -0,0 +1 @@
+* [#12756](https://github.com/rubocop/rubocop/pull/12756): Only parse target Ruby from gemspec if array elements are strings. ([@davidrunger][])

lib/rubocop/target_ruby.rb

@@ -105,7 +105,7 @@ def version_from_gemspec_file(file)
       def version_from_right_hand_side(right_hand_side)
         gem_requirement_versions = gem_requirement_versions(right_hand_side)
 
-        if right_hand_side.array_type?
+        if right_hand_side.array_type? && right_hand_side.children.all?(&:str_type?)
           version_from_array(right_hand_side)
         elsif gem_requirement_versions
           gem_requirement_versions.map(&:value)

spec/rubocop/target_ruby_spec.rb

@@ -188,10 +188,25 @@
 
       context 'when file reads the required_ruby_version from another file' do
         it 'uses the default target ruby version' do
-          content = <<~HEREDOC
+          content = <<~'HEREDOC'
+            Gem::Specification.new do |s|
+              s.name = 'test'
+              s.required_ruby_version = Gem::Requirement.new(">= #{File.read('.ruby-version').rstrip}")
+              s.licenses = ['MIT']
+            end
+          HEREDOC
+
+          create_file(gemspec_file_path, content)
+          expect(target_ruby.version).to eq default_version
+        end
+      end
+
+      context 'when file reads the required_ruby_version from another file in an array' do
+        it 'uses the default target ruby version' do
+          content = <<~'HEREDOC'
             Gem::Specification.new do |s|
               s.name = 'test'
-              s.required_ruby_version = Gem::Requirement.new(">= \#{File.read('.ruby-version').rstrip}")
+              s.required_ruby_version = [">= #{File.read('.ruby-version').rstrip}"]
               s.licenses = ['MIT']
             end
           HEREDOC