Update plv8 to 3.1.8

[bayandin]

Problem

We likely need this to support Postgres 16
It's also been asked by a user #5042

The latest version is 3.2.0, but it require some changes in the build script (which I haven't checked, but it didn't work right away)

Summary of changes

3.1.8       2023-08-01
            - force v8 to compile in release mode

3.1.7       2023-06-26
            - fix byteoffset issue with arraybuffers
            - support postgres 16 beta

3.1.6       2023-04-08
            - fix crash issue on fetch apply
            - fix interrupt issue

From https://github.com/plv8/plv8/blob/v3.1.8/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]

1640 tests run: 1565 passed, 0 failed, 75 skipped (full report)

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Flaky tests (1)

Postgres 14

• test_remote_storage_backup_and_restore[True-local_fs]: release

Code coverage (full report)

• functions: 52.7% (7572 of 14359 functions)
• lines: 81.4% (44565 of 54773 lines)

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_{The comment gets automatically updated with the latest test results
f8c852b at 2023-09-07T11:42:33.817Z :recycle:}

[vadim2404]

does it affect the image size?

[bayandin]

does it affect the image size?

No, it doesn't (for compressed layers):

$  # the latest release:
$ docker manifest inspect neondatabase/compute-node-v14:release-3836 | jq '[.layers[].size] | add' | numfmt --to=si 
257M
$ docker manifest inspect neondatabase/compute-node-v15:release-3836 | jq '[.layers[].size] | add' | numfmt --to=si 
257M
$
$  # current PR:
$  docker manifest inspect neondatabase/compute-node-v14:6108808546 | jq '[.layers[].size] | add' | numfmt --to=si 
257M
$  docker manifest inspect neondatabase/compute-node-v15:6108808546 | jq '[.layers[].size] | add' | numfmt --to=si 
257M