Include letters, numbers & underscore always as Elasticsearch token

What does this MR do?

There are various other regexes here that are trying to capture tokens in different contexts but at the very least we should also always be greedily capturing a series of letters with underscores in it. It's OK if this is already covered in some cases by another regex since we de-duplicate tokens anyway.

The test included in this change is an example where we don't correctly capture this token today and it is a common example in Ruby so we should cover it.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• 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

Related to &3621 (closed)