Add Gemini Flash 2.5 05-20 variants for the Vertex provider (#3758) #7534

Job	Run time
check-openrouter-api-key	1s
check-translations	48s
compile	1m 25s
test-extension (ubuntu-latest)	2m 45s
knip	57s
test-extension (windows-latest)	3m 40s
test-webview (ubuntu-latest)	59s
test-webview (windows-latest)	1m 58s
integration-test	1m 50s
unit-test	1s
	14m 24s

Provide feedback