Seedtag: Sets bid request's currency to USD

[sangarbe]

Forces bid request currency to USD for Seedtag adapter since it's the only supported currency

[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, 17f737e

seedtag

Refer here for heat map coverage report

github.com/prebid/prebid-server/v3/adapters/seedtag/seedtag.go:20:	Builder			100.0%
github.com/prebid/prebid-server/v3/adapters/seedtag/seedtag.go:28:	MakeRequests		95.8%
github.com/prebid/prebid-server/v3/adapters/seedtag/seedtag.go:78:	MakeBids		100.0%
github.com/prebid/prebid-server/v3/adapters/seedtag/seedtag.go:117:	getMediaTypeForBid	100.0%
total:									(statements)		98.0%