[ Cabal ] Removed extensions field

Cabal 1.20.0.2 fixed the issue https://github.com/haskell/cabal/issues/223.
Merge branch 'maint-2.4.0'
[ examples test ] Ulf's DTP08 talk is not outdated
Merge branch 'maint-2.4.0'
[ CHANGELOG ] Fixed Agda version
[ Release notes ] TODO: Agda 2.4.0.X

Before releasing Agda 2.4.2 it is necessary to replace X with the
appropriate value.
[ Release notes ] Stopped talking about Agda 2
Merge branch 'maint-2.4.0'
[ examples ] Fixed Conor's DTP08 talk
Merge branch 'maint-2.4.0'
[ examples ] Fixed `test_files` alphabetic sort
Merge branch 'maint-2.4.0'
[ Makefile ] Added .PHONY targets
Merge branch 'maint-2.4.0'
[ issue 1168 ] Added test case
Merge branch 'issue-1168'
[ Makefile ] Added API tests to the test suit
[ Makefile ] Added API tests to the test suit

Cherry picked from 73769ee6d6c5ecacb76af652eae8fa2c3935b570.
Merge branch 'maint-2.4.0'
[ agda-releases ] Added documentation related to issue 1244