Change beta badge color to something more subtle

[alexfrancoeur]

Related to elastic/kibana#25008 and elastic/kibana#24924

The pink EuiBetaLabel seems to flag both interest and concern. Some users get the "hey let me test that out" feeling and others get the "ahh, I better not touch that" feeling. In order to still call out beta but make it less obtrusive, I'm proposing we change the color to something a bit more subtle. This way when you're using a beta UI regularly, you're not constantly reminded that it is indeed a beta feature.

cc: @tbragin @ryankeairns @snide