build-update-repo: Fix org.telegram.desktop icons

The telegram app id is org.telegram.desktop, and its appstream
component id is org.telegram.desktop.desktop, which we did not
properly handle (we special cased the app-id-ends-with-desktop case
and then did not remote .desktop). This replaces that with a more
approach that *always* matches the whole app id as prefix, and then
replaces a ".desktop" in the suffix part only.

Closes: #1593
Approved by: alexlarsson
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f