[v10.x needs a backport] - assert: improve loose assertion message

[targos]

PR: #22155

/cc @BridgeAR

Feel free to close this issue if the PR should not be backported, but please swap the labels there.

[targos]

@BridgeAR Do you think the PR (or part of it) should be backported?

[BridgeAR]

I guess it would be nice, even though the loose mode is not used that much. I'll have a look into backporting a couple of things today.

[BridgeAR]

Closing as the PR actually relies on a semver-major change.