openwrt-keyring: Only copy sign key for 19.07 and 21.02

openwrt-keyring: Only copy sign key for 19.07 and 21.02

Instead of adding all public signature keys from the openwrt-keyring
repository only add the key which is used to sign the OpenWrt 19.07
feeds and the 21.02 feeds to allow checking the next release.

If one of the other keys would be compromised this would not affect
users of 19.07 release builds.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>