ci: update checkout and cache to v4; runner to ubuntu-latest.

It seems we were using all sorts of versions of these actions throughout
our GA files. v2 is deprecated and I think has been removed entirely.

Then our fuzz runner is set to ubuntu-20.04 which has also been removed.
So update everything to latest. For Rust software there should never be
any surprise breakage due to OS changes. But there has been "surprise
breakage" many times due to removing old versions.

Author: apoelstra

.github/workflows/cron-weekly-update-nightly.yml

@@ -6,7 +6,7 @@ on:
 jobs:
   format:
     name: Update nightly rustc
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@nightly

.github/workflows/fuzz.yml

@@ -6,16 +6,16 @@ jobs:
 
   fuzz:
     if: ${{ !github.event.act }}
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     strategy:
       fail-fast: false
       matrix:
         fuzz_target: [berlekamp_massey, correct_bech32, correct_codex32, decode_rnd, encode_decode, parse_hrp]
     steps:
       - name: Install test dependencies
         run: sudo apt-get update -y && sudo apt-get install -y binutils-dev libunwind8-dev libcurl4-openssl-dev libelf-dev libdw-dev cmake gcc libiberty-dev
-      - uses: actions/checkout@v2
-      - uses: actions/cache@v2
+      - uses: actions/checkout@v4
+      - uses: actions/cache@v4
         id: cache-fuzz
         with:
           path: |
@@ -38,7 +38,7 @@ jobs:
     needs: fuzz
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: actions/download-artifact@v4
       - name: Display structure of downloaded files
         run: ls -R

.github/workflows/rust.yml

@@ -4,7 +4,7 @@ name: Continuous Integration
 
 jobs:
   Prepare:
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-latest
     outputs:
       nightly_version: ${{ steps.read_toolchain.outputs.nightly_version }}
     steps:
@@ -21,7 +21,7 @@ jobs:
       fail-fast: false
     steps:
       - name: Checkout Crate
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Checkout Toolchain
         # https://github.com/dtolnay/rust-toolchain
         uses: dtolnay/rust-toolchain@stable
@@ -39,7 +39,7 @@ jobs:
       fail-fast: false
     steps:
       - name: Checkout Crate
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Checkout Toolchain
         uses: dtolnay/rust-toolchain@beta
       - name: Running test script
@@ -55,7 +55,7 @@ jobs:
       fail-fast: false
     steps:
       - name: Checkout Crate
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Checkout Toolchain
         uses: dtolnay/rust-toolchain@v1
         with:
@@ -74,7 +74,7 @@ jobs:
       fail-fast: false
     steps:
       - name: Checkout Crate
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Checkout Toolchain
         uses: dtolnay/[email protected]
       - name: Running test script
@@ -90,7 +90,7 @@ jobs:
       fail-fast: false
     steps:
       - name: Checkout Crate
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Set up QEMU
         run: sudo apt update && sudo apt install -y qemu-system-arm gcc-arm-none-eabi
       - name: Checkout Toolchain
@@ -112,7 +112,7 @@ jobs:
       fail-fast: false
     steps:
       - name: Checkout Crate
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Set up QEMU
         run: sudo apt update && sudo apt install -y qemu-system-arm gcc-arm-none-eabi
       - name: Checkout Toolchain
@@ -131,7 +131,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout Crate
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Checkout Toolchain
         uses: dtolnay/rust-toolchain@stable
       - name: Add architecture i386
@@ -149,7 +149,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout Crate
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Checkout Toolchain
         uses: dtolnay/rust-toolchain@stable
       - name: Install target