Bump OCaml lower bound to 4.11 and update CI matrix.

OCaml 4.11 is the earliest release currently supported in OPAM.  Earlier
versions may still work but won't be tested.

Author: paurkedal

.github/workflows/workflow.yml

@@ -28,9 +28,10 @@ jobs:
           - "ubuntu"       # libmariadb
           # "ubuntu-18.04" has libmariadbclient but has too old libc for node.js
         ocaml-compiler:
-          - "5.3"
+          - "5.5"
+          - "5.2"
           - "4.14"
-          - "4.07"
+          - "4.11"
 
     runs-on: ubuntu-latest
     container:

mariadb.opam

@@ -18,7 +18,7 @@ build: [
   ]
 ]
 depends: [
-  "ocaml" {>= "4.07.0"}
+  "ocaml" {>= "4.11.0"}
   "ctypes" {>= "0.13.0"}
   "conf-mariadb"
   "conf-gcc"