RFR: 7353: JMC fails to build because it can't download babel dependency

[Alex Macdonald]

This PR addresses JMC-7353 [[0]](https://bugs.openjdk.java.net/browse/JMC-7353) (and JMC-7356 [[1]](https://bugs.openjdk.java.net/browse/JMC-7356)), in which JMC cannot build because the babel dependency is unable to be downloaded: 

[ERROR] All attempts to read artifact osgi.bundle,org.eclipse.compare.core.nl_ja,4.19.0.v20210327020002 failed:
[ERROR] An error occurred while transferring artifact canonical: osgi.bundle,org.eclipse.compare.core.nl_ja,4.19.0.v20210327020002 from repository
https://archive.eclipse.org/technology/babel/update-site/R0.18.3/2021-03


This is due to a recent update to babel for platforms 2020-12 and 2021-03 (and 2021-06, but that doesn't affect us yet ..) [[2]](https://www.eclipse.org/babel/downloads.php), which resulted in a new url being required to fetch the current supported version of babel for 2021-03. This PR updates the url and version of babel for the 2021-03 target platform.

Note that there is also an updated link for the 2020-12 babel, but the older babel isn't causing any problems downloading at the moment, so I've decided to only update the 2021-03 target platform here. 

To verify this you can blow-up your ~/.m2 folder and try a clean install with 2021-03 as the active platform.

[0] https://bugs.openjdk.java.net/browse/JMC-7353
[1] https://bugs.openjdk.java.net/browse/JMC-7356
[2] https://www.eclipse.org/babel/downloads.php

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Commit messages:
 - 7353: JMC fails to build because it can't download babel dependency

Changes: https://git.openjdk.java.net/jmc/pull/281/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=281&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7353
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jmc/pull/281.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/281/head:pull/281

PR: https://git.openjdk.java.net/jmc/pull/281