Merge IdentifierQuoterInterface into QuoterInterface #1206

In #1204, IdentifierQuoterInterface was added with the quoteIdentifier() method. To keep backward compatibility with third-party QuoterInterface implementations, it was introduced as a separate interface instead of adding the method to QuoterInterface directly.

In the next major version:

• Move quoteIdentifier(string $part, string ...$parts): string to QuoterInterface.
• Remove IdentifierQuoterInterface and remove it from the list of interfaces that Quoter implements.
• Remove the instanceof IdentifierQuoterInterface check and the NotSupportedException from
  IdentifierBuilder::build().
• Update references to IdentifierQuoterInterface in PHPDoc and docs.
• Check DBMS-specific packages (db-sqlite, db-mysql, db-pgsql, db-mssql, db-oracle) for references to
  IdentifierQuoterInterface.
• Add an entry to UPGRADE.md that custom QuoterInterface implementations must implement quoteIdentifier().

⚠️ Don't implement it before start 3.0 development