Release v0.3.3

Maintenance update, no functional changes.

- Update Go toolchain base to 1.21.
- Clean up tests and documentation.
- Move the temp marker to the end of the filename.
- cmd/acat: document interrupt behaviour
- Test that panicking in the Tx callback properly calls Cancel.

.github/workflows/go-presubmit.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        go-version: ['stable', 'oldstable']
+        go-version: ['stable']
         os: ['ubuntu-latest']
     steps:
     - uses: actions/checkout@v4