Promote invalid_export_of_internal_element to warning

[greglittlefield-wf]

From #179

This is the hint associated with the @internal annotation from the meta package.

I think we should upgrade this to a warning in the v2 base set of lints, to help prevent accidentally exporting APIs that shouldn't be exported.

[aviary-wf]

Security Insights

No security relevant content was detected by automated scans.

Action Items

• Review PR for security impact; comment "security review required" if needed or unsure
• Verify aviary.yaml coverage of security relevant code

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[evanweible-wf]

QA +1
@Workiva/release-management-p

[rmconsole-wf]

+1 from RM