Fix rendering issue of SunsetPage in dark mode

[bmarty]

Content

Remove the alpha channel of the 2 images: reduce the image size and fixes an issue in dark mode.

Motivation and context

Perfect color transition between the top background (dark color) and the sunset image, see the screenshots below.

Screenshots / GIFs

Before	After

Tests

• Easiest way to test is to force the state in the migration mode, in roomListContentState Composable, set showMigration to true.

Strangely I cannot repro the issue using the Preview, for instance by adding a colored background.

Tested devices

• Physical
• Emulator
• OS version(s):

Checklist

• Changes have been tested on an Android device or Android emulator with API 23
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/element-hq/element-x-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request title will be used in the release note, it clearly define what will change for the user
• Pull request includes screenshots or videos if containing UI changes
• Pull request includes a sign off
• You've made a self review of your PR

[jmartinesp]

I think @ganfra will be very happy with this change!

[github-actions]

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.13%. Comparing base (4aa3961) to head (9308333).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3217   +/-   ##
========================================
  Coverage    76.13%   76.13%           
========================================
  Files         1645     1645           
  Lines        38816    38816           
  Branches      7517     7517           
========================================
  Hits         29554    29554           
  Misses        5362     5362           
  Partials      3900     3900           

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