Adnuntius: Use format=prebidServer on adserver requests

[antosarho]

Instead of format=json, use format=prebidServer in adserver requests.

[github-actions]

Code coverage summary

Note:

• Prebid team doesn't anticipate tests covering code paths that might result in marshal and unmarshal errors
• Coverage summary encompasses all commits leading up to the latest one, adfa65b

adnuntius

Refer here for heat map coverage report

github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius.go:99:	Builder			100.0%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius.go:109:	MakeRequests		100.0%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius.go:113:	setHeaders		100.0%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius.go:129:	makeEndpointUrl		92.9%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius.go:178:	getImpSizes		90.0%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius.go:201:	generateRequests	92.0%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius.go:317:	MakeBids		81.8%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius.go:344:	getSiteExtAsKv		80.0%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius.go:354:	getGDPR			92.9%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius.go:379:	generateReturnExt	86.7%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius.go:413:	generateAdResponse	82.1%
github.com/prebid/prebid-server/v3/adapters/adnuntius/adnuntius.go:486:	generateBidResponse	88.0%
total:									(statements)		89.4%