[Maps] Update map report image test threshold for new EMS styles (#167162)

Updated EMS Styles are waiting to be put into production. They are
already available in Elastic staging environment
([preview](maps-staging.elastic.co/?manifest=testing)). This PR is a
safe measure to ensure that this change do not break our CI tests.

The process has been as follows:

1. Momentarily replaces the EMS Tile Service `tileApiUrl` by our staging
server to force the use of the new styles and check which tests break
with the slightly different basemaps at
[12481c6](12481c6ada)
2. Look for related [broken
tests](https://buildkite.com/elastic/kibana-pull-request/builds/161870)
```
Error: expected 0.030813687704837327 to be below 0.03
```
4. Adjust the threshold for the dashboard report, since the new value
was slightly over the limit
[e655b84](e655b84569)
5. Wait for a green CI (this took a few days because of unrelated issues
with Kibana CI)
6. Revert the `tileApiUrl` change to its original value
[c0030bc](c0030bcff1)

---------

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

x-pack/test/functional/apps/dashboard/group3/reporting/screenshots.ts

@@ -226,7 +226,7 @@ export default function ({
           updateBaselines
         );
 
-        expect(percentDiff).to.be.lessThan(0.03);
+        expect(percentDiff).to.be.lessThan(0.035);
       });
     });
   });