Update macos images for Github Actions to use macos-15

[y9v]

What does this PR do?
This PR updates macos runners to version 15.

Motivation:
The macOS-13 images are being deprecated and jobs that use them are getting automatically cancelled:
https://github.com/DataDog/dd-trace-rb/actions/runs/19269676399/job/55098464682?pr=5038

Change log entry
None. This change is internal.

Additional Notes:
None.

How to test the change?
CI.

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-11-12 09:40:05

Comparing candidate commit aafed90 in PR branch update-macos-images-to-macos-15 with baseline commit c560834 in branch master.

Found 2 performance improvements and 2 performance regressions! Performance is the same for 40 metrics, 2 unstable metrics.

scenario:line instrumentation - targeted

• 🟥 throughput [-8813.880op/s; -8314.366op/s] or [-5.837%; -5.506%]

scenario:line instrumentation - untargeted

• 🟥 throughput [-6590.046op/s; -6468.347op/s] or [-11.081%; -10.876%]

scenario:profiling - sample timeline=false

• 🟩 throughput [+0.385op/s; +0.448op/s] or [+5.685%; +6.613%]

scenario:tracing - Propagation - Datadog

• 🟩 throughput [+3034.440op/s; +3112.721op/s] or [+10.661%; +10.936%]

[datadog-official]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage
• Patch Coverage: 100.00%
• Total Coverage: 98.39% (-0.15%)

View detailed report

_{This comment will be updated automatically if new data arrives.
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[ivoanjo]

👍 Thanks for fixing this one up!