[Curator] Update current version for Curator

[KOTungseth]

Type of issue

Inaccurate

What documentation page is affected

https://www.elastic.co/docs/reference/elasticsearch/curator

What happened?

The Curator landing page in Reference says the current version is 8.0.18, but the current version in the dropdown displays 8.0.21.

Additional info

The version was bumped in elastic/curator#1758

[yetanothertw]

Seems like #1156 is a duplicate.