[chore] Run unit tests for 1.21 (open-telemetry#30324)

I think we weren't running unit tests for Go ~1.21. See
https://github.com/open-telemetry/opentelemetry-collector-contrib/actions/runs/7441862453/job/20244563464?pr=30162
as an example.

.github/workflows/build-and-test.yml

@@ -265,7 +265,7 @@ jobs:
  path: ~/.cache/go-build
  key: go-test-build-${{ runner.os }}-${{ matrix.go-version }}-${{ hashFiles('**/go.sum') }}
  - name: Run Unit Tests
- if: startsWith( matrix.go-version, '1.19' )
+ if: startsWith( matrix.go-version, '~1.21' )
  run: make gotest GROUP=${{ matrix.group }}
  - name: Run Unit Tests With Coverage
  if: startsWith( matrix.go-version, '~1.20' ) # only run coverage on one version