Restore Bundler 1 behavior regarding libraries & gemspecs

Up until the Bundler 2 upgrade, when in the context of updating a
library, dependabot would first try locking the Ruby version according
to the minimum ruby requirement specified in the gemspec (if any). If
that fail, dependabot would retry without any ruby version locking by
detecting a specific error message raised by Bundler. This feature was
introduced by fcd5682.

However, when the upgrade to Bundler 2 happened, the related spec
started failing, because the string to look for within Bundler's error
message was not updated to match what Bundler 2 was raising (error
message was changed upstream at
rubygems/bundler#6647).

Instead, the spec was updated to match the new result (see
dependabot#3319 (comment)).

In the spec, dependabot is updating a Gemfile including

```ruby
gem 'statesman', "~> 3.0.0"
```

in combination with a gemspec including

```ruby
required_ruby_version ">= 1.9.3"
```

Statesman 3.0.0 has a Ruby ">= 2.2" requirement, so it does not support
Ruby 1.9.3 already. The original dependabot behaviour was to ignore the
preexisting mismatch and move on. That makes sense to me, there's some
reasons why a library main have this situation. In my case, I have
several Gemfiles testing different major versions of my dependencies
(Rails, in particular), and some of them don't support the oldest Ruby
supported by my gem (my Rails 7 gemfile does not support my oldest
supported Ruby, 2.6).

On a more pragmatic point of view, the old behavior didn't cause any
reported issues that I know of, while the new behavior did bite my
particular case.

So this commit changes the expectation to what it used to be and updates
the strings to look for in error messages to support both Bundler 1 and
Bundler 2 error messages.

bundler/lib/dependabot/bundler/update_checker/version_resolver.rb

@@ -139,7 +139,8 @@ def error_due_to_restrictive_upper_bound?(error)
         end
 
         def ruby_lock_error?(error)
-          return false unless error.message.include?(" for gem \"ruby\0\"")
+          return false unless error.message.include?(" for the Ruby\0 version") || # Bundler 2
+                              error.message.include?(" for gem \"ruby\0\"") # Bundler 1
           return false if @gemspec_ruby_unlocked
 
           dependency_files.any? { |f| f.name.end_with?(".gemspec") }

bundler/spec/dependabot/bundler/update_checker/version_resolver_spec.rb

@@ -462,15 +462,15 @@
             to eq(Gem::Version.new("2.0.1"))
         end
 
-        context "that isn't satisfied by the dependencies", :bundler_v2_only do
+        context "that isn't satisfied by the dependencies" do
           let(:dependency_files) do
             bundler_project_dependency_files("imports_gemspec_version_clash_old_required_ruby_no_lockfile")
           end
           let(:current_version) { "3.0.1" }
 
-          it "raises a DependencyFileNotResolvable error" do
-            expect { subject }.
-              to raise_error(Dependabot::DependencyFileNotResolvable)
+          it "ignores the minimum ruby version in the gemspec" do
+            expect(resolver.latest_resolvable_version_details[:version]).
+              to eq(Gem::Version.new("7.2.0"))
           end
         end
       end