Postgres basic compatibility versions 13, 14, 15

[mathemancer]

Related to #3199

Gets the tests passing for Postgres major versions 13, 14, and 15. Adds a testing strategy to verify this for those versions.

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[mathemancer]

@silentninja Note that as you suspected, everything was already working. The needed changes were:

• reducing the precision of a check to deal with differing FP errors in v13 vs 14 and 15, and
• Modifying tests to account for the fact that as of v15, you can have precision values greater than scale values for numeric types.

[silentninja]

Thanks! It looks good to me