Add Graphql mutation to update timeline event

What does this MR do and why?

Contributes to #344060 (closed)

Adds GraphQL mutation to update a timeline event.

Query example

mutation UpdateTimelineEvent($input: TimelineEventUpdateInput!) {
  timelineEventUpdate(input: $input) {
    timelineEvent {
      id
      author { id username }
      updatedByUser { id username }
      incident { id title }
      note
      noteHtml
      promotedFromNote { id body }
      editable
      action
      occurredAt
      createdAt
      updatedAt
    }
    errors
  }
}

Query variables

{
  "input": {
    "id": "gid://gitlab/IncidentManagement::TimelineEvent/3",
    "note":"<strong>Updated note</strong>",
    "occurredAt": "2022-01-25T11:38:05Z"
  }
}

Screenshots or screen recordings

Before	Update	After	rails c

How to set up and validate locally

• Navigate to Monitor > Incidents from the left sidebar menu of your GitLab project
• Create a new incident
• Run bundle exec rails c
• Create a new timeline event: IncidentManagement::TimelineEvent.create!(note: "test", note_html: "test", incident: Issue.last, project: Issue.last.project, action: "comment", occurred_at: Time.current)
• Grab the last timeline event ID IncidentManagement::TimelineEvent.last.id
• Open http://localhost:3000/-/graphql-explorer
• Copy & Paste mutation example
• Copy & Paste mutation query variables and update timeline event ID with the last timeline ID from the previous steps
• Run the mutations in GraphiQL
• Check IncidentManagement::TimelineEvent.find(<ID>) in the rails console

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.