RFR: 7510: Remove 2020-06 platform

Fri Jan 14 14:01:28 UTC 2022

On Fri, 14 Jan 2022 13:44:04 GMT, Marcus Hirt <hirt at openjdk.org> wrote:

> Last platform for JDK 8.

lgtm.

Additionally, after the inclusion of the OSX aarch64 environment [[0]](https://github.com/openjdk/jmc/commit/d218205fd4113daea82de1b813d5ef1ed740983d), builds fail on 2021-03 and earlier because 4.20 is required for the aarch64 environment [[1]](https://bugs.openjdk.java.net/browse/JMC-7477). Not sure if you would want to clean up any more platforms here, but 2021-03, 2020-12, and 2020-09 are broken without manual adjustment to remove OSX aarch64.

[0] https://github.com/openjdk/jmc/commit/d218205fd4113daea82de1b813d5ef1ed740983d
[1] https://bugs.openjdk.java.net/browse/JMC-7477

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Marked as reviewed by aptmac (Committer).

PR: https://git.openjdk.java.net/jmc/pull/358