Remove suggestion to wait for Star releases

[zoffixznet]

This was meant to be a PR, but I clicked the wrong button: 166afea

If there's a problem/disagreement with that commit, simply revert it.

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[AlexDaniel]

The change looks correct:

• We should not have many compiler-specific notes
• Any compiler-specific remarks should be marked as such
• If there is something to be said about the version, it should say “starting with rakudo 20xx.xx” or something similar
• So why wait for a star release if required version is clearly marked? It feels like that point was written with pre-Christmas mentality when things were going in and out willy nilly.

[AlexDaniel]

Also, for the most part, I'm only expecting outstanding bugs and quirks in compilers to be mentioned (see #1486). The rest should be documented in terms of v6.c/v6.d and such. I know that this way of doing things is sometimes hard because reasons, but we should be aiming for that consistently.