## Environment
**Liquibase Version**: 4.0.0

**Liquibase Integration & Version**: maven

**Database Vendor & Version**: postgresql 9 running in docker container from dockerHub

**Operating System Type & Version**: macOS Catalina 10.15.6

## Description
for createSequence with xml changeLog file Liquibase uses [AS] syntax when generating SQL. This is not supported by PostgreSQL prior to 10th version
https://docs.liquibase.com/change-types/community/create-sequence.html
https://www.postgresql.org/docs/9.6/sql-createsequence.html vs https://www.postgresql.org/docs/10/sql-createsequence.html

## Steps To Reproduce
clone https://github.com/liquibase/liquibase-test-harness
run ChangeObjectsTest from IDE as JUnit test or from cmd

`mvn failsafe:integration-test -DchangeObjects=createSequence -DinputFormat=xml`
 however input in sql format without `AS int` works
`mvn failsafe:integration-test -DchangeObjects=createSequence -DinputFormat=sql`

or just create xml changeLog with following
`<?xml version="1.0" encoding="UTF-8"?>
<databaseChangeLog
        xmlns="http://www.liquibase.org/xml/ns/dbchangelog"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://www.liquibase.org/xml/ns/dbchangelog
         http://www.liquibase.org/xml/ns/dbchangelog/dbchangelog-3.8.xsd">

```
<changeSet author="oleh" id="1">
    <createSequence dataType="int"
                    incrementBy="1"
                    minValue="1"
                    schemaName="public"
                    sequenceName="test_sequence"
                    startValue="1"/>
    <rollback>
        <dropSequence sequenceName="test_sequence"/>
    </rollback>
</changeSet>
```

</databaseChangeLog>`
and run it agains DB with enough access rights for creating sequences

## Actual Behavior
for postgreSQL <10 it is
`CREATE SEQUENCE  IF NOT EXISTS public.test_sequence AS int START WITH 1 INCREMENT BY 1 MINVALUE 1`

## Expected/Desired Behavior
`CREATE SEQUENCE  IF NOT EXISTS public.test_sequence START WITH 1 INCREMENT BY 1 MINVALUE 1`


public ValidationErrors validate(CreateSequenceStatement statement, Database database, SqlGeneratorChain sqlGeneratorChain) {
        ValidationErrors validationErrors = new ValidationErrors();
        validationErrors.checkRequiredField("sequenceName", statement.getSequenceName());
        validationErrors.checkDisallowedField("startValue", statement.getStartValue(), database, FirebirdDatabase.class);
        validationErrors.checkDisallowedField("incrementBy", statement.getIncrementBy(), database, FirebirdDatabase.class);
        if (isH2WithMinMaxSupport(database)) {
            validationErrors.checkDisallowedField("minValue", statement.getMinValue(), database, FirebirdDatabase.class, HsqlDatabase.class);
            validationErrors.checkDisallowedField("maxValue", statement.getMaxValue(), database, FirebirdDatabase.class, HsqlDatabase.class);
        } else {
            validationErrors.checkDisallowedField("minValue", statement.getMinValue(), database, FirebirdDatabase.class, H2Database.class, HsqlDatabase.class);
            validationErrors.checkDisallowedField("maxValue", statement.getMaxValue(), database, FirebirdDatabase.class, H2Database.class, HsqlDatabase.class);
        }
        validationErrors.checkDisallowedField("ordered", statement.getOrdered(), database, HsqlDatabase.class, PostgresDatabase.class);
        validationErrors.checkDisallowedField("dataType", statement.getDataType(), database, DB2Database.class, HsqlDatabase.class, OracleDatabase.class, MySQLDatabase.class, MSSQLDatabase.class);
        return validationErrors;
    }
    @Override
    public Sql[] generateSql(CreateSequenceStatement statement, Database database, SqlGeneratorChain sqlGeneratorChain) {
        StringBuilder queryStringBuilder = new StringBuilder();
        queryStringBuilder.append("CREATE SEQUENCE ");
        if (database instanceof PostgresDatabase) {
            
            try {
                if (database.getDatabaseMajorVersion() > 9
                        || (database.getDatabaseMajorVersion() == 9 && database.getDatabaseMinorVersion() >= 5)) {
                    queryStringBuilder.append(" IF NOT EXISTS ");
                }
            } catch (DatabaseException e) {
                
            }