data: Enable extension version validation by default again

The default value for the disable-extension-version-validation gsettings key was changed with 5e0e3edc back in 2016. While this turned out to be a good decision for some time, with the last and now upcoming releases bigger changes were and are being done more regularly (various cleanups, removal of Shell.GenericContainer, removal of this.actor), leading to breakage of almost all extensions that modify js-code of the shell.

So switch the default for this gsettings key to false again and leave people with a working but "default" shell experience after updates rather than a broken one.