Enable Ruby deprecation warnings in RSpec

What does this MR do?

Enable deprecation warnings by default and make them more visible to developers to ease upgrading to newer Ruby versions.

Relates to #257438 (comment 447929758)

Screenshots

Running e.g. rspec spec/lib/gitlab/cleanup/project_uploads_spec.rb (taken from #257438 (closed)) without and with deprecation warnings enabled:

Before	After	After with SILENCE_DEPRECATION=1

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• [x ] Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team