feat: backstage v1.26.3

[johanneswuerbach]

https://backstage.io/docs/releases/v1.26.0

Bumped using:

yarn backstage-cli versions:bump
yarn backstage-cli versions:bump --pattern '@{backstage,roadiehq,frontside}/*'
yarn upgrade

The new version requires to set a service-to-service secret https://backstage.io/docs/auth/service-to-service-auth/#legacy-tokens as the Humanitec plugin is still using the legacy backend.

I've tried to also use the new notation with externalAccess, but Backstage won't start with this:

You must configure at least one key in backend.auth.keys for production

As this is only a temporary measure, I don't think it matters a lot.

[mathieu-benoit]

LGTM