spec: allow neon extension auto-upgrade + softfail upgrade

[skyzh]

Problem

reverts #7128, unblocks https://github.com/neondatabase/cloud/issues/10742

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]

2748 tests run: 2609 passed, 0 failed, 139 skipped (full report)

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Code coverage* (full report)

• functions: 28.2% (6331 of 22469 functions)
• lines: 47.0% (44525 of 94665 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
186a34b at 2024-03-28T17:36:18.903Z :recycle:}

[save-buffer]

Is there a simple way to add a test to this? If it's not easy to do then maybe not worth it to add one, but just a thought

[skyzh]

Is there a simple way to add a test to this? If it's not easy to do then maybe not worth it to add one, but just a thought

Could be part of the compatibility test, I can give a try.

[skyzh]

added test case in e6cbf6b 402829d

[save-buffer]

Nice!