Use larger builders for CPU-bound workflows (#2256)

- `ubuntu-22.04-firezone` is a 16-core builder for ~~kotlin~~ and docker
- ~~`macos-13-xlarge` is an M1 builder for Apple and docker arm64~~
- Configure the Gradle build cache
- Upgrade kotlin plugins, Android minSDK to 30, and gradle to 8.4


Edit: It appears that even using the largest runners for kotin and swift
don't speed the builds up that much (~30%), but will substantially
increase our cost, so I've reverted them to free.

Fixes #2210

.github/workflows/ci.yml

@@ -24,7 +24,7 @@ jobs:
   # less overhead to keep things in GH actions. See work on building these
   # in GCP with Cloud Build: https://github.com/firezone/firezone/pull/2234
   build-images:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04-firezone-16c
     strategy:
       matrix:
         include:

.github/workflows/elixir.yml

@@ -9,7 +9,7 @@ concurrency:
 
 jobs:
   unit-test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04-firezone-4c
     defaults:
       run:
         working-directory: ./elixir
@@ -414,7 +414,7 @@ jobs:
             env.cache-name }}-${{ hashFiles('**/elixir/mix.lock') }}
 
   acceptance-test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04-firezone-4c
     defaults:
       run:
         working-directory: ./elixir

.github/workflows/kotlin.yml

@@ -10,11 +10,9 @@ concurrency:
 jobs:
   # See rust.yml how we build, package and release connlib as an example
   build:
-    runs-on: ${{ matrix.runs-on }}
-    strategy:
-      matrix:
-        runs-on:
-          - ubuntu-latest
+    # Builds are more IO-bound than CPU-bound so using a larger GitHub-hosted
+    # runner isn't worth it.
+    runs-on: ubuntu-latest
     defaults:
       run:
         working-directory: ./kotlin/android
@@ -31,7 +29,7 @@ jobs:
       - uses: actions/setup-java@v3
         with:
           distribution: temurin
-          java-version: 11
+          java-version: 17
       - name: Validate Gradle wrapper
         uses: gradle/wrapper-validation-action@v1
       - name: Create Local Properties File
@@ -43,7 +41,7 @@ jobs:
       - name: Run linter
         run: ./gradlew spotlessCheck
       - name: Execute Gradle build
-        run: ./gradlew build
+        run: ./gradlew assembleRelease
       - name: Run Test
         run: ./gradlew testReleaseUnitTest
       - name: Android Test Report

.github/workflows/swift.yml

@@ -14,6 +14,8 @@ jobs:
       fail-fast: false
       matrix:
         runs-on:
+          # Builds are more IO-bound than CPU-bound so using a larger GitHub-hosted
+          # runner isn't worth it.
           - os: macos-13
             xcode-ver: "14.3"
         target: