RFR: 7794: Update the link to the adoptium download site

[Bernd]

The AdoptOpenJDK no longer has the latest builds.
Also use https for Links.

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Commit messages:
 - Update README.md
 - README.md: Update Link to Eclipse download site

Changes: https://git.openjdk.java.net/jmc/pull/397/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=397&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7794
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jmc/pull/397.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/397/head:pull/397

PR: https://git.openjdk.java.net/jmc/pull/397