resolution_test.py uses deprecated GDK function

### Bug Description

On a recent Noble SRU test: https://certification.canonical.com/hardware/202008-28109/submission/463479/

the resolution_test.py job failed, and while looking into that I noticed the deprecation warning. This should be addressed to clear these warnings (noise) from the results and also to be ready for when this function goes away.

/tmp/nest-l_nt8jg2.6e83b59da52f9107948b3064f37269c15a2f7b95244dd45ed99038120adfb1b6/resolution_test.py:24: DeprecationWarning: Gdk.Screen.get_primary_monitor is deprecated
geom = screen.get_monitor_geometry(screen.get_primary_monitor())
/tmp/nest-l_nt8jg2.6e83b59da52f9107948b3064f37269c15a2f7b95244dd45ed99038120adfb1b6/resolution_test.py:24: DeprecationWarning: Gdk.Screen.get_monitor_geometry is deprecated
geom = screen.get_monitor_geometry(screen.get_primary_monitor())

### Cert-blocker Test Case

- [ ] cert-blocker

### To Reproduce

run sru and the resolution test
notice deprecation warnings

### Expected Result

we are not using deprecated functions, or if older versions of GDK still need to be supported, the code handles both cases (tests for new versions and then falls back to the old one when needed).

### Actual Result

currently we call a function throwing a deprecation warning

### Environment

SRU: stock noble linux-generic
Jenkins URL: http://10.102.156.15:8080/job/cert-stock-sru-noble-desktop-prodesk-400-g7-sku3-28109/37/

### Relevant log output

```shell

```

### Additional context

_No response_

Start with resolution_test.py, especially line 24, and inspect the GDK calls producing the deprecation warnings. Run the resolution test in the Noble SRU environment to confirm the current warnings. Done means the test avoids deprecated functions while preserving support for older GDK versions if required.

Written by the indexing model from the issue text.