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

[Marcus Hirt]

> Also cleaning up the maven versioning.

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

  Using the flattening plug-in in the application as well

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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/15cf33eb..3e30c869

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jmc&pr=206&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jmc&pr=206&range=02-03

  Stats: 25 lines in 1 file changed: 25 ins; 0 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