[EOL] Ubuntu 20.04 is no longer updated

[jmservier]

Checklist

• I've looked through the issues and pull requests for similar reports

Describe your issue

Ubuntu 20.04 reach is EOL on may 2025 we have to switch to another LTS for security purpose at least.
impacted file: https://github.com/cross-rs/cross/blob/main/docker/Dockerfile.x86_64-unknown-linux-gnu

What target(s) are you cross-compiling for?

x86_64-unknown-linux-gnu

Which operating system is the host (e.g computer cross is on) running?

• macOS
• Windows
• Linux / BSD
• other OS (specify in description)

What architecture is the host?

• x86_64 / AMD64
• arm32
• arm64 (including Mac M1)

What container engine is cross using?

• docker
• podman
• other container engine (specify in description)

cross version

0.2.5

Example

No response

Additional information / notes

No response

[Emilgardis]

See #1580