Use coveralls.io instead of codecov.io

Codecov.io has changed their permissions in a way that is incompatible
with the org this repo is hosted in; switch to coveralls.io.

.github/workflows/ci.yaml

@@ -103,11 +103,8 @@ jobs:
         uses: taiki-e/install-action@cargo-llvm-cov
       - name: Generate code coverage
         run: cargo llvm-cov --all-features --workspace --exclude xtask --lcov --output-path lcov.info
-      - name: Upload coverage to Codecov
-        uses: codecov/codecov-action@v3
-        with:
-          files: lcov.info
-          fail_ci_if_error: true
+      - name: Coveralls
+        uses: coverallsapp/github-action@v2
 
   spellcheck:
     runs-on: ubuntu-22.04

README.md

@@ -1,6 +1,6 @@
 # gpt-disk-rs
 
-[![codecov](https://codecov.io/gh/google/gpt-disk-rs/branch/main/graph/badge.svg?token=qN5MUcTVQ5)](https://codecov.io/gh/google/gpt-disk-rs)
+[![Coverage Status](https://coveralls.io/repos/github/google/gpt-disk-rs/badge.svg?branch=main)](https://coveralls.io/github/google/gpt-disk-rs?branch=main)
 
 `no_std` libraries related to [GPT] (GUID Partition Table) disk
 data. There are three Rust packages in this repository: