ref(inbox): use new issue-inbox flag on backend

[malwilley]

#122110 must be merged and deployed first

Drops issue-stream-progress-ui for the new issue-inbox flag

[sentry]

Bug: Removing the issue-stream-progress-ui feature flag will cause the inbox page to render a NotFound error because the frontend still relies on this flag to render.
_{Severity: CRITICAL}

Suggested Fix

This change has a deployment dependency on a frontend pull request. Ensure the frontend PR, which removes the dependency on the issue-stream-progress-ui flag, is merged and deployed before this backend change. Alternatively, coordinate the deployment of both PRs to happen simultaneously to avoid breaking the inbox page.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: src/sentry/features/temporary.py#L382

Potential issue: The pull request removes the backend registration for the
`organizations:issue-stream-progress-ui` feature flag. However, the frontend code in
`static/app/views/issueList/pages/inbox.tsx` checks for the presence of this flag to
render the inbox page. If this backend change is deployed before the corresponding
frontend change that removes this check, the condition
`organization.features.includes('issue-stream-progress-ui')` will always be false. This
will cause the component to return a `NotFound` page, making the entire inbox feature
inaccessible to all users.

Also affects:

• static/app/views/issueList/pages/inbox.tsx:151

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[malwilley]

This is expected, the other PR changes the frontend pieces

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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