Casts EXTRACT result as double precision in preparation for PG 14

What does this MR do and why?

Overview

As the EXTRACT function now returns numeric instead of double precision in PG14, these changes will ensure compatibility when we bump PG version.

As Rails treats numeric value as BigDecimal, .to_json and .as_json will cast BigDecimal as a string. This behavior could lead to issues, because theses values will be represented as String, instead of Float.

PG 13

[
  {
    avg: -24.2183307368421053
  }
]

PG 14

[
  {
    avg: "-24.2183307368421053"
  }
]

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MR acceptance checklist

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Related to #398772 (closed)