Resolve "Unify es_type as symbol or string"

What does this MR do?

Issue #197878 (closed)

Change usage of the es_type field in all method parameters, checks, and tests to use Strings instead of Strings or Symbols. Since es_type returns strings in all models, string was chosen as the standard. Removed any checks that converted to symbols (and back to string for pluralization). Changed all method param defaults and tests so that strings are used.

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

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• 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
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