Use poetry 2.1 (#1803)

The pyproject.toml file requires `poetry>=2.1`, but the
`poetry-plugin-export` package version was pulling a lower value, which
masked the fact that we had some old syntax sticking around for
poetry<2.1. The recent Dependabot PR to bump `dulwich` (#1801 ) exposed
this mess. This PR bumps the appropriate packages and cleans up the old
poetry code that is no longer needed in CI. It also upgrades PennyLane
to 0.45.0 in the project build itself, which was missed post-release.

Build tested
[here](https://github.com/PennyLaneAI/demos/actions/runs/26641980390) ✅

Author: doctorperceptron

.github/workflows/v2-build-demos.yml

@@ -181,7 +181,7 @@ jobs:
           python-version: '3.11'
 
       - name: Install dependencies
-        run: pip install . && poetry config warnings.export false
+        run: pip install .
 
       - name: Make demo name list
         id: demo-name-list

.github/workflows/v2-sync-objects-dot-inv.yml

@@ -137,7 +137,7 @@ jobs:
           python-version: '3.11'
 
       - name: Install dependencies
-        run: pip install . && poetry config warnings.export false
+        run: pip install .
 
       - name: Build all demos in HTML format
         run: |