PG17 compatibility: add helper function for EXPLAIN diffs in scalar subquery output

[colm-mchugh]

PG17 changed how scalar subquery outputs appear in EXPLAIN output (*). This commit changes impacted regress goldfiles to the PG17 format, and adds a helper function to covert pre-PG17 plans to the PG17 format. The conversion is required when testing Citus on pgversions prior to 17. The helper function can and should be removed when 17 becomes the minimum supported version.

(*) https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=fd0398fcb

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## release-13.0 #7757 +/- ## =============================================== Coverage ? 89.64% =============================================== Files ? 274 Lines ? 59583 Branches ? 7436 =============================================== Hits ? 53413 Misses ? 4038 Partials ? 2132 ```

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