Stop postgres in immediate mode in VM image.

[arssher]

PR #6712 might add a delay in graceful shutdown without change to compute -> sk protocol to force sync of control file on safekeeper to persist commit_lsn, so use 'immediate' mode instead; 'fast' one isn't useful for neon anyway.

[github-actions[bot]]

2436 tests run: 2319 passed, 0 failed, 117 skipped (full report)

---

Flaky tests (2)

Postgres 16

• test_create_snapshot: debug

Postgres 15

• test_create_snapshot: debug

Test coverage report is not available

_{The comment gets automatically updated with the latest test results
bce8d4b3f5a94791876ed4caae366d94fbc08b49 at 2024-02-14T17:38:58.487Z :recycle:}

[bayandin]

I've noticed that we also use fast mode in tests by default: https://github.com/neondatabase/neon/blob/568bc1fde3f770aa8e1fd0dc8128a7add779a29f/test_runner/fixtures/neon_fixtures.py#L3168

Should we change it to immediate as well?