Remove ethers-solc

ethers-solc was used for a type (now manually specified) and to call out to
solc. Since Foundry was already a documented dependency, a call to it now
handles building.

Removing this single crate removes a total of 17 crates from our dependency
tree. While these may still be around due to Foundry, they at least may not
be.

Further work to remove the requirement on Foundry for solc alone would be
appreciated.

.github/actions/build-dependencies/action.yml

@@ -54,5 +54,11 @@ runs:
         components: ${{ inputs.rust-components }}
         targets: wasm32-unknown-unknown, riscv32imac-unknown-none-elf
 
+    - name: Install Foundry
+      uses: foundry-rs/foundry-toolchain@cb603ca0abb544f301eaed59ac0baf579aa6aecf
+      with:
+        version: nightly
+        cache: false
+
     # - name: Cache Rust
     #   uses: Swatinem/rust-cache@a95ba195448af2da9b00fb742d14ffaaf3c21f43