Make the version a private class property

Problem/Motivation

In #3089902: "Azure Database for MySQL server" reports wrong database version we added

  protected function getServerVersion(): string {
    static $server_version;
    if (!$server_version) {
      $server_version = $this->connection->query('SELECT VERSION()')->fetchColumn();
    }
    return $server_version;
  }

@neclimdul rightly points out that if a database connects to mutliple mysql databases of different versions this potentially could cause you problems.

Proposed resolution

Make it a private class property. Original this was inlined as a static because having to select the server version from the database and not get it from PDO is entirely caused by proxy sql services reporting incorrect versions and it felt wrong to add class properties to work around there bugs - but now we've introduced bugs of our own :(

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet

N/a