RFR: 8210: Update devguide to mention Protocol Rules for Eclipse 2023-06 onward

[Alex Macdonald]

This PR adds some information to the devguide about using Eclipse 2023-06 and above. Newer Eclipse IDEs force the use of HTTPS for external update sites and artifacts, which out of the box conflicts with our local p2 repository.

The fix is to allow http in Preferences > Install/Update > Trust > Authorities > Protocol Rules. I've added a screenshot to doc as well highlighting where to find it. 

I had run into this yesterday when taking a look at some interesting behaviour I was seeing when testing the Jolokia PR https://github.com/openjdk/jmc/pull/548#issuecomment-1989331436.

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Commit messages:
 - 8210: Update devguide to mention Protocol Rules for Eclipse 2023-06 onward

Changes: https://git.openjdk.org/jmc/pull/556/files
  Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=556&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8210
  Stats: 4 lines in 2 files changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jmc/pull/556.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/556/head:pull/556

PR: https://git.openjdk.org/jmc/pull/556