Support H2 1.4 & 2.0 in H2SequenceMaxValueIncrementer

Prior to this commit, H2SequenceMaxValueIncrementer only supported H2
database 1.4.

This commit updates H2SequenceMaxValueIncrementer's getSequenceQuery()
method so that the syntax used supports version 1.4 and 2.0 of the H2
database.

This commit also updates several test schemas so that they work with H2
1.4 and 2.0 as well as HSQL.

Closes gh-27870

spring-r2dbc/src/test/resources/org/springframework/r2dbc/connection/init/users-schema.sql

@@ -1,7 +1,7 @@
 DROP TABLE users IF EXISTS;
 
 CREATE TABLE users (
-	id INTEGER NOT NULL IDENTITY,
+	id INTEGER GENERATED BY DEFAULT AS IDENTITY,
 	first_name VARCHAR(50) NOT NULL,
 	last_name VARCHAR(50) NOT NULL
 );