spec: disable neon extension auto upgrade

[skyzh]

Problem

This pull request disables neon extension auto upgrade to help the next compute image upgrade smooth.

Summary of changes

We have two places to auto-upgrade neon extension: during compute spec update, and when the compute node starts. The compute spec update logic is always there, and the compute node start logic is added in #7029. In this pull request, we disable both of them, so that we can still roll back to an older version of compute before figuring out the best way of extension upgrade-downgrade. #6936

We will enable auto-upgrade in the next release following this release.

There are no other extension upgrades from release 4917 and therefore after this pull request, it would be safe to revert to release 4917.

Impact:

• Project created after unpinning the compute image -> if we need to roll back, they will stuck, because the default neon extension version is 1.3. Need to manually pin the compute image version if such things happen.
• Projects already stuck on staging due to not downgradeable -> I don't know their current status, maybe they are already running the latest compute image? They are already pinned to release-5037 and we should not change the pinning status of these projects.
• Other projects -> can be rolled back to release 4917.

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

[skyzh]

Release plan:

• Release this image so that everyone has downgrade script, and release the week after next week to upgrade everyone to 1.3.
• For compute nodes not waking up during the following week between {this downgradeable relese, next upgrade release}, they might be stuck. Need run check_availability on all existing computes.

[github-actions]

2652 tests run: 2528 passed, 0 failed, 124 skipped (full report)

---

Flaky tests (1)

Postgres 15

• test_ondemand_activation: debug

Code coverage* (full report)

• functions: 28.4% (7032 of 24719 functions)
• lines: 47.2% (43469 of 92116 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
2d2d6fa at 2024-03-14T15:07:15.269Z :recycle:}