set xmlns for sitemap-news also with gnews_single mode

[hannob]

This should fix the errors I reported here:
https://board.s9y.org/viewtopic.php?f=3&t=24196

The problem in the code is this: The sitemap xmlns was included conditionally if $gnewsmode is set. However that is only the case for the separate gnews sitemap.
In the case of gnews_single - where the news items are included in the main sitemap - it's not. But it is still required.

There's a check for that and it sets $this->gnewsmode, but after that block, so I had to switch those code blocks around and change the if($gnewsmode) to if($this->gnewsmode).

Please merge.

[yellowled]

@hannob Thx! 👍

[onli]

In fact I missed something: Changes like this should always also raise the version number. Especially important for spartacus plugins to trigger upgrades. I will do that in a separate commit now.

[yellowled]

@onli Can you please also document @hannob s changes in NEWS while you're at it?

[onli]

Do we document plugin changes there? But I added a line to the plugin changelog: ca327e3

[yellowled]

No, I just totally missed that this is in additional_plugins, sorry.

[onli]

I did at first as well^^ This plugin might as well be a core plugin.