fix(unparser): use to_rfc3339 for default TIMESTAMPTZ formatting

[sgrebnov]

Which issue does this PR close?

• Closes unparser: use to_rfc3339 for TIMESTAMPTZ formatting #21294.

Rationale for this change

The default timestamp_with_tz_to_string uses DateTime<Tz>.to_string(), which produces literals like 2025-09-15 11:00:00 +00:00 that are not supported by multiple dialects (e.g. DuckDB, BigQuery). Switching to to_rfc3339() produces valid ISO 8601 / RFC 3339 strings (e.g. 2025-09-15T11:00:00+00:00) that are broadly compatible.

What changes are included in this PR?

• Changed the default Dialect::timestamp_with_tz_to_string implementation from dt.to_string() to dt.to_rfc3339()
• Removed now-redundant timestamp_with_tz_to_string overrides from DuckDBDialect and BigQueryDialect (they produced equivalent RFC 3339-compatible output via custom format strings)
• Updated test expectations to match the new default format

Are these changes tested?

Yes, existing tests in unparser::expr::tests have been updated to reflect the new format.

Manually verifying updated format supported by PostgreSQL, MySQL, SQLite, DuckDB and BigQuery.

Are there any user-facing changes?

Yes — timestamp-with-timezone literals are now formatted as RFC 3339 (2025-09-15T11:00:00+00:00) instead of the previous Display format (2025-09-15 11:00:00 +00:00). Dialects that need a different format can still override timestamp_with_tz_to_string.

[AndreaBozzo]

Seems solid, waiting for someone else to take another look

[nuno-faria]

Thanks @sgrebnov.