Codecov pull request checks: use non-deprecated v2 version of action

As documented on https://github.com/codecov/codecov-action, v1 of
Codecov's GitHub action will cease to function on 2022-02-01. Given that
we do not seem to use any advanced flags, the migration should not
require changes.

It does, however, permit one change: as CBMC is a public project, a
token isn't even necessary for the upload.

Author: tautschnig

.github/workflows/pull-request-checks.yaml

@@ -706,9 +706,8 @@ jobs:
           lcov --capture --directory build --output-file lcov.info
           lcov --remove lcov.info '/usr/*' --output-file lcov.info
       - name: Upload coverage statistics to Codecov
-        uses: codecov/codecov-action@v1
+        uses: codecov/codecov-action@v2
         with:
-          token: ${{ secrets.CODECOV_TOKEN }}
           files: ./lcov.info
           fail_ci_if_error: true
           verbose: true