Add next Rust minor to test matrix (#794)

Automation to ensure we test on all supported Rust versions as new stable Rust versions are released.

The following is the output from `git diff`:

```diff
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 4cd52df..a202c7b 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -157,7 +157,7 @@ jobs:
         #
         #            Also make sure to update the MSRV in the cargo-semver-checks-action CI:
         #            https://github.com/obi1kenobi/cargo-semver-checks-action/blob/main/.github/workflows/test-action.yml#L18
-        toolchain: ["1.74", "1.75", "1.76", "1.77", "stable", "beta"]
+        toolchain: ["1.74", "1.75", "1.76", "1.77", "1.78", "stable", "beta"]
         experimental: [false]
         include:
           - toolchain: "nightly"
```

Co-authored-by: github-actions <github-actions@github.com>

.github/workflows/ci.yml

@@ -157,7 +157,7 @@ jobs:
         #
         #            Also make sure to update the MSRV in the cargo-semver-checks-action CI:
         #            https://github.com/obi1kenobi/cargo-semver-checks-action/blob/main/.github/workflows/test-action.yml#L18
-        toolchain: ["1.74", "1.75", "1.76", "1.77", "stable", "beta"]
+        toolchain: ["1.74", "1.75", "1.76", "1.77", "1.78", "stable", "beta"]
         experimental: [false]
         include:
           - toolchain: "nightly"