Update LibreOffice Viewer to 7.6.0.0.alpha0+/d6c54b3d4ee7

Update LibreOffice Viewer to the current development version (git master branch).

Changes as compared to last existing entry (versionName 6.1.0.0.alpha0+/484d0ea842da, versionCode 15) besides the LibreOffice version update are listed in more detail in the commit message.

Required

• The app complies with the inclusion criteria
• All related fdroiddata and RFP issues have been referenced in this merge request
• Builds with fdroid build

Strongly Recommended

• The upstream app source code repo contains the app metadata (summary/description/images/changelog/etc) in a Fastlane or Triple-T folder structure
• Releases are tagged

Suggested

• External repos are added as git submodules instead of srclibs
• Multiple apks for native code

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Closes #1923 (closed)