snap: update requirements of Bitcoin Core 28.0

[EndrII]

Bitcoin core at now require libc 2.31 to tests,

• changed build image to Ubuntu 20.04 (It's still deprecated, but we can't move to 24.04 because bitcoin core app still depends on qt5)
• changed snap core depends on Ubuntu 20 (it's not important but better if build environment will be same as runtime environment)

[maflcko]

Ref: #248 (comment)

Also, please squash your commits according to https://github.com/bitcoin/bitcoin/blob/master/CONTRIBUTING.md#squashing-commits

[EndrII]

Also, please squash your commits according to https://github.com/bitcoin/bitcoin/blob/master/CONTRIBUTING.md#squashing-commits

done

[maflcko]

The unity7 note can be removed from the pull description.

[maflcko]

See #248 (comment) about the CI failure

[EndrII]

It seems that all efforts should be made to change the image where snapd is already installed. I will continue to work on this later.

[maflcko]

I tried something in #253, but that didn't work.

[EndrII]

I tried something in #253, but that didn't work.

You tried to run build with multipass (default) it's wrong, it will not work, because multipass works as general VM with kvm system.

i think we should try this. This is an official docer container from canonical with snapcraft