Update Rust version to v1.81.0 in GitHub workflows (#65)

Update rust toolchain version in ci.yml, rustdoc.yml to v1.81.0

Author: thearossman

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
     - name: Install Rust stable
       uses: actions-rs/toolchain@v1
       with:
-        toolchain: 1.75.0
+        toolchain: 1.81.0
         override: true
         components: rustfmt, clippy
     - name: Set up Python ${{ matrix.python-version }}

.github/workflows/rustdoc.yml

@@ -16,7 +16,7 @@ jobs:
     - name: Install Rust stable
       uses: actions-rs/toolchain@v1
       with:
-        toolchain: 1.75.0
+        toolchain: 1.81.0
         override: true
         components: rustfmt, clippy
     - name: Set up Python ${{ matrix.python-version }}