Fix table comment setting in Iceberg tables

[dlouseiro]

resolves https://github.com/dbt-labs/dbt-adapters/issues/770

Problem

Persisting table comments in Iceberg tables fails as comment on ... is ... notation does not work for Iceberg tables.

Solution

• Update snowflake__alter_relation_comment to set relation_type = 'iceberg table' when the is_iceberg_format is True.
• Change code to use alter <relation type> ... set comment = ... notation instead of comment on ... is ... notation

Checklist

• [x] I have read the contributing guide and understand what's expected of me
• [x] I have run this code in development and it appears to resolve the stated issue
• [x] This PR includes tests, or tests are not required/relevant for this PR
• [x] This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

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[dlouseiro]

Closing in favour of this one in the monorepo