RFR: 7324: Update eclipse version and Min JDK required in Update Site landing sites

[Guru Hb]

With Eclipse 4.19 JMC can't be used with Boot JDK 8. Currently the Update sites landing page and README.txt had instruction on how JMC can be used with JDK8 (Which is not valid any more). 

Updated instruction to use JMC with JDK 11 and above.

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Commit messages:
 - 7324: Update eclipse version and Min JDK required in Update Site landing pages

Changes: https://git.openjdk.java.net/jmc/pull/278/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=278&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7324
  Stats: 18 lines in 7 files changed: 1 ins; 0 del; 17 mod
  Patch: https://git.openjdk.java.net/jmc/pull/278.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/278/head:pull/278

PR: https://git.openjdk.java.net/jmc/pull/278