[ci] Auto-trigger uitests and device-tests on darc-* branches (dotnet#34317)

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### Description of Change

Add `darc-*` to the `trigger: branches: include:` section in
`ci-uitests.yml` and `ci-device-tests.yml` so that `maui-pr-uitests` and
`maui-pr-devicetests` automatically run when dotnet-maestro pushes
dependency updates to `darc-*` branches.

Previously, these pipelines required manual `/azp run` comments on every
maestro PR.

### Issues Fixed

N/A - CI improvement

### Files Changed

- `eng/pipelines/ci-uitests.yml` - Added `darc-*` to CI trigger branch
filter
- `eng/pipelines/ci-device-tests.yml` - Added `darc-*` to CI trigger
branch filter

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <[email protected]>

Author: 3 people

eng/pipelines/ci-device-tests.yml

@@ -5,6 +5,7 @@ trigger:
     - release/*
     - net*.0
     - inflight/*
+    - darc-*
   tags:
     include:
     - '*'

eng/pipelines/ci-uitests.yml

@@ -5,6 +5,7 @@ trigger:
     - release/*
     - net*.0
     - inflight/*
+    - darc-*
   tags:
     include:
     - '*'