imds Bid Adapter : fix incorrectly named user sync property

[ecammit]

Type of change

• Bugfix

Description of change

• This changes the "image" usersync from the incorrectly used "pixel" property. This was preventing the new image user sync from happening and causing a warning in the console with debug enabled of: WARNING: User sync type "pixel" not supported.
• Removed a warning being emitted for valid 204 no-bid responses with an empty body.
• There is also a change to the markdown documentation to match imds: Update Google Ad Manager video tag information prebid.github.io#4908

Other information

prebid/prebid.github.io#4908

[ecammit]

Note: Failing circleci is from some unrelated bid adapter (lm_kiviadsBidAdapter) that's consistently failing.