fix(releases): tweak text styling on chart release label (#50658)

before:
<img width="108" alt="Screenshot 2023-06-12 at 6 32 00 PM"
src="https://github.com/getsentry/sentry/assets/1976777/44b057bc-697f-4b66-8d93-33861c36db5e">
<img width="77" alt="Screenshot 2023-06-12 at 6 31 30 PM"
src="https://github.com/getsentry/sentry/assets/1976777/5837054a-95fe-43f8-874a-c1a5f9d572d0">
<img width="107" alt="Screenshot 2023-06-12 at 6 31 22 PM"
src="https://github.com/getsentry/sentry/assets/1976777/9e3c70c2-f8c5-45a0-b9cd-dd768152de21">

after:
<img width="112" alt="Screenshot 2023-06-12 at 6 32 24 PM"
src="https://github.com/getsentry/sentry/assets/1976777/11d44306-b217-41ab-8b65-d42b6c0c48b9">

<img width="105" alt="Screenshot 2023-06-12 at 6 32 15 PM"
src="https://github.com/getsentry/sentry/assets/1976777/dd0607c5-4c91-422c-bdfc-058fcbb28283">
<img width="121" alt="Sc
<img width="157" alt="Screenshot 2023-06-12 at 6 30 37 PM"
src="https://github.com/getsentry/sentry/assets/1976777/209973f9-adb6-4978-ac94-2c13da1d8788">


<img width="121" alt="Screenshot 2023-06-12 at 6 30 44 PM"
src="https://github.com/getsentry/sentry/assets/1976777/bea3ec46-bf11-4a92-a00a-73543f85a0e2">

static/app/views/releases/detail/utils.tsx

@@ -179,7 +179,9 @@ function generateReleaseMarkLine(
         formatter: hideLabel ? '' : title,
         // @ts-expect-error weird echart types
         font: 'Rubik',
-        fontSize: 11,
+        fontSize: 14,
+        color: theme.chartLabel,
+        backgroundColor: theme.chartOther,
       },
       data: [
         {