Fix Extensions Index CMakeLists.txt checks

[JonaRichter]

Summary

• Fix EXTENSION_SCREENSHOTURLS format: the Slicer Extensions Index checker requires the set() call on a single line with no linebreak before the closing quote/paren (set(EXTENSION_SCREENSHOTURLS "...")). The previous multi-line format (both as 4 separate quoted URLs and as a single wrapped string) failed this check with "No EXTENSION_SCREENSHOTURLS found in CMakeLists.txt."
• Update EXTENSION_ICONURL and all EXTENSION_SCREENSHOTURLS to point to the main branch instead of the no-longer-existing master branch, so the referenced images actually resolve.
• Remove the dead master branch trigger entries in .github/workflows/ci.yml (main was already covered; master no longer exists).

Test plan

• Confirm Extensions Index automated check passes on the referenced CMakeLists.txt content