Update Nextcloud to 26 and instructions

[Vdauphin]

First of all, thank you for your contribution! 😄

Please note that this repo is mostly for popular apps with thousands of stars and tens of thousands of downloads. If you'd like to add a less popular app, you can always create your own 3rd party repo and add your app there.

☑️ Self Check before Merge

• I have tested the template using the method described in README.md thoroughly
• I have ensured that I put as much default values as possible (except passwords) to ensure minimum effort required for end users to get started.
• I have ensured that I am not using the "latest" tag as this tag is dynamically changing and might break the one-click app. Use a fixed version.
• I have made sure that instructions.start and instructions.end are clear and self-explanatory.
• Icon is added as a png file to the logos directory.
• I've executed the checks if necessary by running npm ci && npm run validate_apps && npm run formatter
• I will take responsibility addressing any issues that arises as a result of this PR (maintaining this app).

[githubsaturn]

Thanks!