Search replacement for swagger-validator-action

[zehnm]

The GitHub action mbowman100/swagger-validator-action@master used to validate OpenAPI seems unmaintained. It's only a matter of time until GitHub disables the set-output command and the action breaks. I've opened an issue: mbowman100/swagger-validator-action#16

We should search for an alternative if the action is no longer maintained, e.g. https://github.com/char0n/swagger-editor-validate seems to be up to date.