Clean up FormatSystemMethods quoter

[jzabroski]

• Firebird changed how utc support works in v4.0
• MySQL 5.5 supports UTC_TIMESTAMP https://dev.mysql.com/doc/refman/5.5/en/date-and-time-functions.html#function_utc-timestamp
• and so on

[jzabroski]

Something like this:

src\FluentMigrator.Runner.Db2\Generators\Db2\Db2Quoter.cs

        {
            switch (value)
            {
                case SystemMethods.NewSequentialId:
                case SystemMethods.NewGuid:
                    return base.FormatSystemMethods(value);
                case SystemMethods.CurrentUTCDateTime:
                    /*
                     * (The CURRENT TIMEZONE value is determined from C runtime functions.)
                     * Source: https://www.ibm.com/support/knowledgecenter/en/SSEPGG_9.7.0/com.ibm.db2.luw.sql.ref.doc/doc/r0005887.html
                     */
                    return "(CURRENT_TIMESTAMP - CURRENT_TIMEZONE)";
                case SystemMethods.CurrentDateTime:
                    return "CURRENT_TIMESTAMP";

src\FluentMigrator.Runner.Firebird\Generators\Firebird\FirebirdQuoter.cs

                case SystemMethods.NewSequentialId:
                case SystemMethods.NewGuid:
                    return "gen_uuid()";
                case SystemMethods.CurrentDateTime:
                    return "CURRENT_TIMESTAMP";
                case SystemMethods.CurrentUser:
                    return "CURRENT_USER";
                /*
                 Firebird v.3.0.4: Context variables LOCALTIME and LOCALTIMESTAMP were added as synonyms for CURRENT_TIME and CURRENT_TIMESTAMP, respectively, in anticipation of CURRENT_TIME and CURRENT_TIMESTAMP being redefined in Firebird 4 to represent the time and timestamp in UTC time, to comply with the SQL standards.
                 Source: https://firebirdsql.org/file/documentation/release_notes/html/en/3_0/bk02ch09s06.html


                 Source: https://github.com/FirebirdSQL/firebird/blob/master/doc/sql.extensions/README.time_zone.md#changes-in-current_time-and-current_timestamp
                 */
                case SystemMethods.CurrentDateTime:
                    return "LOCALTIMESTAMP";
                case SystemMethods.CurrentDateTimeOffset:
                    throw new ArgumentException($"{nameof(SystemMethods.CurrentDateTimeOffset)} not supported.");
                case SystemMethods.CurrentUTCDateTime:
                    return "CURRENT_TIMESTAMP";
            }

src\FluentMigrator.Runner.MySql\Generators\MySql\MySqlQuoter.cs

namespace FluentMigrator.Runner.Generators.MySql
@ -51,6 +53,8 @@ namespace FluentMigrator.Runner.Generators.MySql
                    return "UTC_TIMESTAMP";
                case SystemMethods.CurrentUser:
                    return "CURRENT_USER()";
                case SystemMethods.CurrentDateTimeOffset:
                    throw new ArgumentException($"{nameof(SystemMethods.CurrentDateTimeOffset)} not supported.");
            }