docs(api): hide API reference entries for post-2.20 features (#16304)

# Overview

🤫 Hides 2.21 features from the 2.20 docs.

## Test Plan and Hands on Testing


[Sandbox](http://sandbox.docs.opentrons.com/docs-hide-post-2.20-ref-entries/v2/new_protocol_api.html)

Things you shouldn't see:
- `RobotContext` stuff
- Future liquid level stuff

## Changelog

- `:meta private:` tags for `ProtocolContext.robot` and `Well.meniscus`
- Bumped the required version for `ProtocolContext.robot` because it's
not useful in 2.20.

## Review requests

OK with the version change? It will still be 2.20 in the release branch.
I can revert that line if we prefer.

## Risk assessment

v low

api/src/opentrons/protocol_api/labware.py

@@ -229,6 +229,8 @@ def meniscus(self, z: float = 0.0) -> Location:
         :return: A :py:class:`~opentrons.types.Location` corresponding to the
             absolute position of the meniscus-center of the well, plus the ``z`` offset
             (if specified).
+
+        :meta private:
         """
         return Location(self._core.get_meniscus(z_offset=z), self)
 

api/src/opentrons/protocol_api/protocol_context.py

@@ -209,8 +209,12 @@ def api_version(self) -> APIVersion:
         return self._api_version
 
     @property
-    @requires_version(2, 20)
+    @requires_version(2, 21)
     def robot(self) -> RobotContext:
+        """The :py:class:`.RobotContext` for the protocol.
+
+        :meta private:
+        """
         return self._robot
 
     @property