Downgrade plv8 for Postgres 14/15

[bayandin]

Problem

Backport #5318

Summary of changes

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

2484 tests run: 2363 passed, 0 failed, 121 skipped (full report)

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Flaky tests (8)

Postgres 16

• test_crafted_wal_end[last_wal_record_crossing_segment]: debug
• test_partial_evict_tenant: release, debug
• test_pg_regress: debug
• test_pageserver_restarts_under_worload: debug
• test_get_tenant_size_with_multiple_branches: release
• test_wal_lagging: release

Postgres 14

• test_get_tenant_size_with_multiple_branches: debug

Code coverage (full report)

• functions: 53.0% (7672 of 14463 functions)
• lines: 81.0% (44819 of 55362 lines)

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_{The comment gets automatically updated with the latest test results
8686ab6 at 2023-09-15T17:31:07.681Z :recycle:}