Limelight New Adapter: pgamrtb (#6469)

* Remove required host from Limelight prebid server adapters

* revert changes

* revert changes

* new adapter pgamrtb

* Update headings in pgamrtb.md for consistency

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Co-authored-by: apykhteyev <alex@project-limelight.com>
Co-authored-by: belousovdmitry <belousov.dmitry92@gmail.com>
Co-authored-by: maksimD24 <129727954+maksimD24@users.noreply.github.com>
Co-authored-by: mderevyanko <mderevyanko24@gmail.com>