Use macos-latest instead of ubuntu-latest in GitHub workflows

Tycho 4.x requires Maven 3.9.0 or later.

`ubuntu-latest` currently provides Maven 3.8.8, while `macos-latest`
provides Maven 3.9.5.

In addition, macOS virtual machines currently have 3 processors and 14
GB of memory, while Linux virtual machines have 2 processors and 7 GB of
memory:

https://docs.github.com/en/actions/using-jobs/choosing-the-runner-for-a-job#choosing-github-hosted-runners

.github/workflows/ci-build.yml

@@ -5,7 +5,7 @@ on: [push, pull_request]
 jobs:
   build-latest-target:
 
-    runs-on: ubuntu-latest
+    runs-on: macos-latest
 
     steps:
     - uses: actions/checkout@v4
@@ -31,7 +31,7 @@ jobs:
 
   build-base-target:
 
-    runs-on: ubuntu-latest
+    runs-on: macos-latest
 
     steps:
     - uses: actions/checkout@v4

.github/workflows/release.yml

@@ -8,7 +8,7 @@ on:
 jobs:
   build-and-release:
 
-    runs-on: ubuntu-latest
+    runs-on: macos-latest
 
     steps:
     - uses: actions/checkout@v4