[BugFix] Add variant to non reserved keywords (backport #72356)

[mergify]

Why I'm doing:

The new Variant type is not listed as a non reserved keyword like all the other type keywords. Because of this, we were encountering parsing errors when materialized views ran that had a struct field named Variant. This issue persisted in created materialized views and surfaced when they ran not on creation. The issue remained even when we escaped the word variant with backticks.

What I'm doing:

This PR adds variant as a non-reserved keyword. This fixed the issue for us.

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• Interface/UI changes: syntax, type conversion, expression evaluation, display information
• Parameter changes: default values, similar parameters but with different default values
• Policy changes: use new policy to replace old one, functionality automatically enabled
• Feature removed
• Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
  • This pr needs auto generate documentation
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • 4.1
  • 4.0
  • 3.5
  • 3.4

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This is an automatic backport of pull request #72356 done by [Mergify](https://mergify.com).