Update macro regexp_instr.sql for Athena support

[tom-orvn-al]

Add Athena support to the regex macro 'expect_column_values_to_match_regex'

Summary of Changes

• Added athena__regexp_instr macro implementation to support Athena/Trino databases
• Implemented regex matching using Athena's native regexp_like function instead of unsupported regexp_instr
• Added proper warning handling for unsupported is_raw and flags parameters on Athena
• Maintains compatibility with existing test configurations while enabling Athena users to use regex validation tests

Why Do We Need These Changes

The dbt_expectations.expect_column_values_to_match_regex test currently fails on Athena/Trino databases with the error: FUNCTION_NOT_FOUND: Function 'regexp_instr' not registered

This is because Athena/Trino doesn't support the regexp_instr function used by the default implementation. This PR enables dbt-expectations users on Athena to perform regex validation on their data quality tests, which is essential for validating formats like email addresses, phone numbers, and other pattern-based data.

Testing

• Tested with email regex validation: '^.+@.+\..+$'
• Verified backward compatibility with other database adapters
• Confirmed proper handling of unsupported parameters (is_raw, flags)

Reviewers

@tpoll @gusvargas