Use date range for iteration display and make title optional

Proposal

1. Make iteration title optional
2. Use iteration dates as the primary attributes for display (when listing/showing iterations)

Why?

The idea of iteration (or sprint) is allowing measurements of team velocity and capacity through timeboxing as @cwoolley-gitlab describes well in this note #349537 (comment 801322905)

In other words, the "categorization" or "naming" of groups of issues is unrelated and orthogonal to iterations - that is what labels, epics, and other semantic grouping approaches are used for.

In fact, we observed that users often embed dates and team names in iterations titles as a hack to make iterations readily searchable and distinguishable. The cadence feature currently in development will provide the ability to group (done in !63643 (merged)) and find (WIP #336611 (closed)) iterations by their cadences.

Implementation tasks

• [-] backend Make iteration sequence attribute available via GraphQL and REST API

  (update: we won't make use of iteration sequence. See: #349537 (comment 803716187))

• frontend Update various iteration components to use date range when displaying iterations

  • !78039 (merged): updates iteration lists
  • !78361 (merged): updates iteration dropdowns
  • The iteration notification message needs to be updated to include cadence (the task is tracked separately in #350318 (closed)).

• [-] backend Make iteration an optional attribute !78044 (closed) To be tracked in #353350 (closed)

• [-] frontend Update the iteration creation/edit form to indicate title is optional (we are deprecating iteration titles).

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