[Release] Parse OV version on demand (#3247)

### Changes

Follow-up to #3233 . Fix an issue arising during documentation building.

### Reason for changes

Caught here:
https://github.com/openvinotoolkit/nncf/actions/runs/13131916886/job/36638718655
. This test is run on PRs to develop only. That's why it was not caught
during merging to release branch.

### Tests

https://github.com/openvinotoolkit/nncf/actions/runs/13132576726/job/36640708447?pr=3246

nncf/common/utils/backend.py

@@ -22,7 +22,6 @@
     import openvino  # type: ignore # noqa: F401
 
     _OPENVINO_AVAILABLE = True
-    _OPENVINO_VERSION = version.parse(openvino.__version__.split("-")[0])
 except ImportError:
     _OPENVINO_AVAILABLE = False
 
@@ -189,4 +188,5 @@ def is_openvino_at_least(version_str: str) -> bool:
     if not _OPENVINO_AVAILABLE:
         return False
 
-    return version.parse(version_str) <= _OPENVINO_VERSION
+    openvino_version = version.parse(openvino.__version__.split("-")[0])
+    return version.parse(version_str) <= openvino_version