RFR: 7267: Change artifact id from "org.openjdk.jmc.agent" to "agent"

[Patrick Reinhart]

- Changes the current artifact id from `org.openjdk.jmc.agent` to `agent`
- Changes all agent artifact/version references within command line arguments accordingly

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Commit messages:
 - 7267: Change artifact id from "org.openjdk.jmc.agent" to "agent"

Changes: https://git.openjdk.java.net/jmc/pull/261/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=261&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7267
  Stats: 13 lines in 1 file changed: 1 ins; 1 del; 11 mod
  Patch: https://git.openjdk.java.net/jmc/pull/261.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/261/head:pull/261

PR: https://git.openjdk.java.net/jmc/pull/261