[GHC] #16338: Update CI images to 8.6.* and cabal-2.4.*

[GHC]

#16338: Update CI images to 8.6.* and cabal-2.4.*
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           Reporter:  mpickering     |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The docker images need updating to a more recent GHC release. Probably
 best to do this after 8.6.4 is released.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16338>
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