WalProposerLib was dependent on PG16 specifically, so revert its update to PG17

[MMeent]

Problem

Builds on main are broken

Summary of changes

This reverts WPL to use PG16, which I expect will fix the issue of https://github.com/neondatabase/neon/actions/runs/10839581462/job/30080277772

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

[orca-security-us]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca

[github-actions]

No tests were run or test report is not available

Test coverage report is not available

_{The comment gets automatically updated with the latest test results
1391f90 at 2024-09-12T23:22:45.104Z :recycle:}