RFR: 7116: Update version to 8.1.0 in master [v5]

[Marcus Hirt]

> Also cleaning up the maven versioning.

Marcus Hirt has updated the pull request incrementally with one additional commit since the last revision:

  Moving the plug-in xml

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Changes:
  - all: https://git.openjdk.java.net/jmc/pull/206/files
  - new: https://git.openjdk.java.net/jmc/pull/206/files/3e30c869..ba212246

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jmc&pr=206&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jmc&pr=206&range=03-04

  Stats: 50 lines in 1 file changed: 25 ins; 25 del; 0 mod
  Patch: https://git.openjdk.java.net/jmc/pull/206.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/206/head:pull/206

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