feat(dashboards): Replace assignee selector with only avatar

[scttcper]

This is the last place we're using the old assignee selector. It overflows the widget. This might not be the best place to assign an issue.

before - with assignee selector open

Instead, render just the current assignee. This will also stop displaying the old stacked recommended assignee.

after

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
8574	1	8573	2

View the top 1 failed tests by shortest run time

getIssueFieldRenderer Issue fields can render assignee

Stack Traces | 1.04s run time

Error: Unable to find a label with the text of: Assigned to Test User

Ignored nodes: comments, script, style
...
    at waitForWrapper (.../sentry/sentry/node_modules/@.../dom/dist/wait-for.js:163:27)
    at .../sentry/sentry/node_modules/@.../dom/dist/query-helpers.js:86:33
    at Object.<anonymous> (.../utils/dashboards/issueFieldRenderers.spec.tsx:97:48)
    at Promise.then.completed (.../sentry/sentry/node_modules/jest-circus/build/utils.js:298:28)
    at new Promise (<anonymous>)
    at callAsyncCircusFn (.../sentry/sentry/node_modules/jest-circus/build/utils.js:231:10)
    at _callCircusTest (.../sentry/sentry/node_modules/jest-circus/build/run.js:316:40)
    at processTicksAndRejections (node:internal/process/task_queues:105:5)
    at _runTest (.../sentry/sentry/node_modules/jest-circus/build/run.js:252:3)
    at _runTestsForDescribeBlock (.../sentry/sentry/node_modules/jest-circus/build/run.js:126:9)
    at _runTestsForDescribeBlock (.../sentry/sentry/node_modules/jest-circus/build/run.js:121:9)
    at _runTestsForDescribeBlock (.../sentry/sentry/node_modules/jest-circus/build/run.js:121:9)
    at run (.../sentry/sentry/node_modules/jest-circus/build/run.js:71:3)
    at runAndTransformResultsToJestFormat (.../sentry/sentry/node_modules/jest-circus/build/legacy-code-todo-rewrite/jestAdapterInit.js:122:21)
    at jestAdapter (.../sentry/sentry/node_modules/jest-circus/build/legacy-code-todo-rewrite/jestAdapter.js:79:19)
    at runTestInternal (.../sentry/sentry/node_modules/jest-runner/build/runTest.js:367:16)
    at runTest (.../sentry/sentry/node_modules/jest-runner/build/runTest.js:444:34)
    at Object.worker (.../sentry/sentry/node_modules/jest-runner/build/testWorker.js:106:12)

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[codecov]

Bundle Report

Changes will increase total bundle size by 780.67kB (2.49%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
app-webpack-bundle-array-push	32.07MB	780.67kB (2.49%) ⬆️

[codecov]

Bundle Report

Changes will increase total bundle size by 780.45kB (2.49%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
app-webpack-bundle-array-push	32.07MB	780.45kB (2.49%) ⬆️