GHA: Add OCaml 5.3 to the build matrix #3997

Job	Run time
Analyse	35s
Build-macOS (4.14.2)	2m 51s
Build-macOS (5.3.0)	2m 43s
Cygwin	5s
Hygiene	48s
Build-Linux (4.08.1)	2m 7s
Test-macOS (4.14.2)	24m 46s
Build-Linux (4.09.1)	2m 15s
Build-Linux (4.10.2)	2m 14s
Build-Linux (4.11.2)	2m 11s
Test-macOS (5.3.0)	31m 46s
Build-Linux (4.12.1)	3m 47s
Build-Linux (4.13.1)	2m 25s
Build-Linux (5.0.0)	2m 27s
Build-Linux (5.1.1)	2m 6s
Build-Linux (5.2.0)	2m 6s
Build-Linux (4.14.2)	2m 4s
Build-Linux (5.3.0)	2m 13s
Build-Windows (x86_64-pc-cygwin, x86_64-pc-cygwin, 4.14.2)	36m 6s
Build-Windows (i686-w64-mingw32, x86_64-pc-cygwin, 4.14.2)	40m 25s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 4.14.2)	40m 38s
Build-Windows (i686-pc-windows, x86_64-pc-cygwin, 4.14.2)	45m 33s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 4.14.2)	42m 37s
Build-Windows (x86_64-pc-cygwin, x86_64-pc-cygwin, 5.3.0)	26m 27s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 5.3.0)	40m 24s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 5.3.0)	42m 9s
Upgrade-macOS (4.14.2)	2m 1s
Solvers-macOS (z3, 4.14.2)	15m 19s
Upgrade-macOS (5.3.0)	2m 4s
Solvers-macOS (z3, 5.3.0)	14m 18s
Solvers-macOS (0install, 4.14.2)	7m 31s
Solvers-macOS (0install, 5.3.0)	7m 12s
Cold-Linux	6m 3s
Doc-Linux (4.14.2)	4m 22s
Test-Linux (4.14.2)	14m 0s
Solvers-Linux (z3, 4.14.2)	16m 19s
Doc-Linux (5.3.0)	5m 37s
Test-Linux (5.3.0)	11m 57s
Upgrade-Linux (4.14.2)	1m 20s
Solvers-Linux (z3, 5.3.0)	17m 9s
Upgrade-Linux (5.3.0)	1m 20s
Solvers-Linux (0install, 4.14.2)	5m 41s
Solvers-Linux (0install, 5.3.0)	5m 48s
	9h 1m 49s

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