Bump the requirement for dune to 2.8 #659

Job	Run time
Analyse	5s
Build-macOS (4.14.1)	2m 9s
Test-macOS (4.14.1)	26m 0s
Hygiene	20s
Build-Linux (4.08.1)	1m 55s
Cygwin	4s
Build-Linux (4.09.1)	2m 3s
Build-Linux (4.10.2)	1m 51s
Build-Linux (4.11.2)	1m 51s
Build-Linux (4.12.1)	1m 43s
Build-Linux (4.13.1)	1m 47s
Build-Linux (5.0.0)	1m 53s
Build-Linux (5.1.1)	1m 46s
Build-Linux (4.14.1)	1m 47s
Build-Windows (x86_64-pc-cygwin, x86_64-pc-cygwin, 4.14.1)	28m 4s
Build-Windows (i686-w64-mingw32, x86_64-pc-cygwin, 4.14.1)	34m 1s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 4.14.1)	35m 9s
Build-Windows (i686-pc-windows, x86_64-pc-cygwin, 4.14.1)	39m 57s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 4.14.1)	42m 28s
Cold-Linux	4m 35s
Test-Linux (4.14.1)	13m 45s
Upgrade-Linux (4.14.1)	1m 9s
Solvers-Linux (z3, 4.14.1)	15m 38s
Solvers-Linux (0install, 4.14.1)	5m 31s
Upgrade-macOS (4.14.1)	1m 50s
Solvers-macOS (z3, 4.14.1)	12m 0s
Solvers-macOS (0install, 4.14.1)	5m 31s
	4h 44m 52s

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