Add extra stop-words for greek language [LUCENE-9329]

Very minor but it would be nice to add a few more useful **stop-words** for the **_greek_** language. So far, I came up with the ones below:

**no** further _formatting_ is done here
```
στα
στουσ
στισ
τισ
στων
```
As per the documentation, I believe the new words need to be appended in [lucene/analysis/el/stopwords.txt](https://github.com/apache/lucene-solr/blob/master/lucene/analysis/common/src/resources/org/apache/lucene/analysis/el/stopwords.txt)

I will find some time and create a PR for this.

Thanks

---
Migrated from [LUCENE-9329](https://issues.apache.org/jira/browse/LUCENE-9329) by Christos Liontos

Open lucene/analysis/common/src/resources/org/apache/lucene/analysis/el/stopwords.txt and review the existing Greek stop-word entries and formatting. Append the five listed words in the file's established format; done means all five entries are present without extra formatting changes.

Written by the indexing model from the issue text.