Update uptime-kuma to version 1.23.15

[nmfretz]

🤖 This is an automated pull request to update uptime-kuma to version 1.23.15. This PR must be reviewed and tested before merging.

[nmfretz]

🤖 This is an automated summary of installation tests for uptime-kuma:

• ✅ App successfully installed in umbrel-dev VM
• ✅ App successfully listed in umbrel.yaml
• ⚠️ App UI accessibility was not tested

This PR must still be reviewed and tested before merging.

[github-actions[bot]]

🎉   Linting finished with no errors or warnings   🎉

Thank you for your submission! This is an automated linter that checks for common issues in pull requests to the Umbrel App Store.

Please review the linting results below and make any necessary changes to your submission.

Linting Results

Severity	File	Description
ℹ️	uptime-kuma/docker-compose.yml	Mounted file/directory "/uptime-kuma/data/uptime-kuma" doesn't exist: The volume "${APP_DATA_DIR}/data/uptime-kuma:/app/data" tries to mount the file/directory "/uptime-kuma/data/uptime-kuma", but it is not present. This can lead to permission errors!
ℹ️	uptime-kuma/docker-compose.yml	Potentially using unsafe user in service "server": The default container user "root" can lead to security vulnerabilities. If you are using the root user, please try to specify a different user (e.g. "1000:1000") in the compose file or try to set the UID/PUID and GID/PGID environment variables to 1000.

Legend

Symbol	Description
❌	Error: This must be resolved before this PR can be merged.
⚠️	Warning: This is highly encouraged to be resolved, but is not strictly mandatory.
ℹ️	Info: This is just for your information.

[nmfretz]

reviewed and tested