Added gem metadata and corrected links in gemspec.yml

[kpumuk]

Currently, while gemspec.yml includes metadata links for documentation, issue tracker, and other, the section is not used in the gemspec. This change corrects that.

In addition, I have noticed that the links are incorrect and point to no longer relevant repository name bundler-audit.rb.

Difference on RubyGems

On the RubyGems page, most of the links were inferred from homepage, so there is a link to source code and documentation and the only on missing is "Changelog".

Metadata difference

To test, I built the gem with rake build and compared the listing before and after the change:

tar -xOzf pkg/bundler-audit-0.9.2.gem metadata.gz | gunzip

Full diff:

--- before.txt	2024-09-12 07:08:08
+++ after.txt	2024-09-12 07:07:32
@@ -129,6 +129,10 @@
 licenses:
 - GPL-3.0-or-later
 metadata:
+  documentation_uri: https://rubydoc.info/gems/bundler-audit
+  source_code_uri: https://github.com/rubysec/bundler-audit
+  bug_tracker_uri: https://github.com/rubysec/bundler-audit/issues
+  changelog_uri: https://github.com/rubysec/bundler-audit/blob/master/ChangeLog.md
   rubygems_mfa_required: 'true'
 post_install_message:
 rdoc_options: []