[Yaml] inline syntax: remove object support

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #6498
| License       | MIT
| Doc PR        |

Since #11976, the ``!!float`` syntax is used to tag whole number floats. So, it's probably not a good idea to deprecate the ``!!`` completely anymore. What do you think? Do you have any idea how to do this properly?