chore(nimbus): bump kinto test server from 30.1.1 to latest

[jaredlockhart]

Summary

The local Kinto test server used for integration tests (mozilla/remote-settings:30.1.1) is very outdated — the latest is v42.3.0.

Firefox 149's Remote Settings client now sends _since timestamps as bare integers (v2 format), but Kinto v1 expects quoted strings and returns HTTP 400: "The value should be integer between double quotes." This broke the desktop enrollment integration tests.

A workaround was added in #15032 (clear local RS DB before each sync), but bumping to a v2-compatible Kinto server would fix this properly and allow removing the workaround.

Acceptance Criteria

• Bump mozilla/remote-settings image in docker-compose.yml to latest stable version
• Update kinto/server.ini if the config format changed
• Verify all integration tests pass with the new server
• Remove the client.db.clear() workaround from conftest.py

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