Use variable for default branch instead of hardcoding main (#184)

We still use `master` several places, so need to use the default branch variable.

.github/workflows/dependency-review.yaml

@@ -59,5 +59,5 @@ jobs:
           allow-licenses: 0BSD, Apache-2.0, BlueOak-1.0.0, BSD-2-Clause, BSD-2-Clause-FreeBSD, BSD-3-Clause, CC-BY-3.0, CC-BY-4.0, CC0-1.0, ISC, LGPL-2.1, MIT, MIT-0, MPL-2.0, ODC-By-1.0, OFL-1.1, Python-2.0, Unicode-DFS-2016, Unlicense, WTFPL, Zlib, (MIT OR Apache-2.0) AND Unicode-DFS-2016, Apache-2.0 AND BSD-3-Clause, ISC AND MIT, MIT AND Zlib, MIT AND BSD-3-Clause, MIT AND WTFPL
           allow-ghsas: ${{ inputs.allow-ghsas }}
           allow-dependencies-licenses: ${{ inputs.allow-dependencies-licenses }}
-          base-ref: ${{ inputs.base-ref || github.event.pull_request.base.sha || 'main' }}
+          base-ref: ${{ inputs.base-ref || github.event.pull_request.base.sha || github.event.repository.default_branch }}
           head-ref: ${{ inputs.head-ref || github.event.pull_request.head.sha || github.ref }}