refactor: close button - WPB-15211

[caldrian]

WPB-15211 [iOS] Update the UI for backup export

Issue

This PR moves an icon for the close button from WireDesign to WireReusableUIComponents.
Moreover, the accessibilityLabel argument is replaced by a standard view modifier.

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Checklist

• Title contains a reference JIRA issue number like [WPB-XXX].
• Description is filled and free of optional paragraphs.
• Adds/updates automated tests.

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UI accessibility checklist

If your PR includes UI changes, please utilize this checklist:

• Make sure you use the API for UI elements that support large fonts.
• All colors are taken from WireDesign.ColorTheme or constructed using WireDesign.BaseColorPalette.
• New UI elements have Accessibility strings for VoiceOver.

[datadog-wireapp]

Datadog Report

Branch report: refactor/close-button-WPB-15211
Commit report: d64d22b
Test service: wire-ios-mono

✅ 0 Failed, 1758 Passed, 27 Skipped, 2m 58.08s Total Time

[github-actions]

Test Results

    2 files     11 suites   3m 6s ⏱️
1 867 tests 1 840 ✅ 27 💤 0 ❌
1 878 runs  1 851 ✅ 27 💤 0 ❌

Results for commit de88a3c.

[samwyndham]

I think the intention here of having the accessibilityLabel as part of the CloseButton came from Agis - the idea was to force us to always use an accessibilityLabel.

I'm OK to remove it, but I can't really think of any cons keeping it.