RFR: 7116: Update version to 8.1.0 in master [v4]

[Jie Kang]

On Mon, 25 Jan 2021 15:36:25 GMT, Marcus Hirt <hirt at openjdk.org> wrote:

>> core/pom.xml line 274:
>> 
>>> 272: 			<plugin>
>>> 273: 				<groupId>org.codehaus.mojo</groupId>
>>> 274: 				<artifactId>flatten-maven-plugin</artifactId>
>> 
>> @thegreystone 
>> Can the flatten-maven-plugin also be used for jmc too? The artifacts produced for jmc-core look good now, but the ones produced for jmc have m2 pom files with ${revision}${changelist}.
>
> Yes, certainly. Added. Let me know if it works the way you want.

This was added to the `pluginManagement` section so it will configure plugins for projects down the tree but not actually be active. I think it should be moved to the `plugins` section one level up within `build`.

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PR: https://git.openjdk.java.net/jmc/pull/206