Make SQL::Abstract work.

This removes the optional dependence on SQL::Abstract::Classic or a
version of SQL::Abstract prior to 2.000000, and only uses version
2.000000 or newer of SQL::Abstract.

For this to work the capability of a field override needs to be removed.
There doesn't seem to be a reliable way to do that with the newer
versions of SQL::Abstract, although I didn't try very hard.  I also
don't see the need for having a field override.  The only field that has
an override is the `key_not_a_keyfield` or `key` field.  Why was that
ever done?  Did someone think that MySQL would make a field a database
key field if it had the name "key"?  Did someone think it would be
confusing to have a field named "key" that was not a database key field?
It was probably the latter, but in my opinion the field override was
even more confusing.

The `key` field in the `key` table does not contain any permanent data,
so upgrading courses does not require any special handling.  The data
from the old `key_not_a_keyfield` field can be safely dropped, and the
new `key` field used in its place.  You can leave the
`key_not_a_keyfield` field in the database if you want to switch back
and forth between other pull requests that use the `key_not_a_keyfield`
field.

bin/check_modules.pl

@@ -141,6 +141,7 @@ =head1 DESCRIPTION
 	Scalar::Util
 	SOAP::Lite
 	Socket
+	SQL::Abstract
 	Statistics::R::IO
 	String::ShellQuote
 	SVG
@@ -166,7 +167,8 @@ =head1 DESCRIPTION
 	'LWP::Protocol::https' => 6.06,
 	'Mojolicious'          => 9.22,
 	'Net::SSLeay'          => 1.46,
-	'Perl::Tidy'           => 20220613
+	'Perl::Tidy'           => 20220613,
+	'SQL::Abstract'        => 2.000000
 );
 
 my ($test_programs, $test_modules, $show_help);
@@ -249,29 +251,6 @@ sub check_modules {
 			print "   $module found and loaded\n";
 		}
 	}
-	checkSQLabstract();
-}
-
-## this is specialized code to check for either SQL::Abstract or SQL::Abstract::Classic
-
-sub checkSQLabstract {
-	print "\n checking for SQL::Abstract\n\n";
-	eval "use SQL::Abstract";
-	my $sql_abstract         = not($@);
-	my $sql_abstract_version = $SQL::Abstract::VERSION if $sql_abstract;
-
-	eval "use SQL::Abstract::Classic";
-	my $sql_abstract_classic = not($@);
-
-	if ($sql_abstract_classic) {
-		print qq/ You have SQL::Abstract::Classic installed. This package will be used if either
- the installed version of SQL::Abstract is version > 1.87 or if that package is not installed.\n/;
-	} elsif ($sql_abstract && $sql_abstract_version <= 1.87) {
-		print "You have version $sql_abstract_version of SQL::Abstract installed.  This will be used\n";
-	} else {
-		print qq/You need either SQL::Abstract version <= 1.87 or need to install SQL::Abstract::Classic.
- If you are using cpan or cpanm, it is recommended to install SQL::Abstract::Classic.\n/;
-	}
 }
 
 1;