feat(audit): add grade-based events

Summary

• feat(audit): remove irrelevant fields from log entries

Now the records look like this:

{
  "message": "instance is starting",
  "severity": "low",
  "time": "2023-11-22T22:10:24.211+0300",
  "title": "local_startup"
}
{
  "message": "target grade of instance `i1` changed to Online(7)",
  "severity": "low",
  "time": "2023-11-22T22:10:24.334+0300",
  "title": "change_target_grade"
}
{
  "message": "current grade of instance `i1` changed to Replicated(7)",
  "severity": "medium",
  "time": "2023-11-22T22:10:24.436+0300",
  "title": "change_current_grade"
}

• feat(audit): add grade-based events

This patch adds the following events:

• create_database
• change_target_grade
• change_current_grade

Part of #349 (closed)

Ensure that

• New code is covered by tests
• API is documented
• Changelog is up to date
• (if Lua API changed) Lua API version is bumped in luamod.rs
• A follow-up doc issue: docs#129 (closed)