upgrade default neon version to 1.6

[myrrc]

Changes for 1.6 were merged and deployed two months ago https://github.com/neondatabase/neon/blob/main/pgxn/neon/neon--1.6--1.5.sql. In order to deploy https://github.com/neondatabase/neon/pull/12183, we need 1.6 to be default, otherwise we can't use prewarm API on read-only replica (ALTER EXTENSION won't work) and we need it for promotion

[github-actions[bot]]

8481 tests run: 7899 passed, 0 failed, 582 skipped (full report)

---

Flaky tests (1)

Postgres 17

• test_sharding_split_failures[failure6]: release-arm64-with-lfc

Code coverage* (full report)

• functions: 32.1% (9014 of 28066 functions)
• lines: 48.4% (80101 of 165443 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
11957cfae6932a8d626b08c0a6f316352e062f1d at 2025-06-11T13:11:12.314Z :recycle:}

[MMeent]

Fail-proof rollback code has not been released yet (introduced only with #12202), so I don't think we can safely default to 1.6 yet.

[myrrc]

Agreed, let's wait till next release