Update Open-webUI to v0.5.20

[beecho01]

Tested by updating app-store files on Umbrel OS 1.4, and causing update to trigger:

• open-webui/docker-compose.yml
• open-webui/umbrel-app.yml

Update was successful without issues and existing data persists.

[github-actions]

🎉   Linting finished with no errors or warnings   🎉

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Linting Results

Severity	File	Description
ℹ️	open-webui/docker-compose.yml	Potentially using unsafe user in service "web": 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]

Thanks @beecho01! Tested and working well. I added in release notes from 0.5.19 as well since we missed that update.