ci: update checkout and cache to v4; runner to Ubuntu 25.04.

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 25.04 which should give us several years of room.
(Maybe we should just switch to ubuntu-latest?)

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-25.04
     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-25.04
     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-25.04
     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/rust-toolchain@1.56.1
       - 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