chore: drop version from README.md which is not really useful (dashpay#5811)

## What was done?
drop version from README.md which is not really useful.

And we will care about one less thing during each release

## Breaking Changes
N/A

## Checklist:
- [x] I have performed a self-review of my own code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have added or updated relevant unit/integration/functional/e2e
tests
- [ ] I have made corresponding changes to the documentation
- [x] I have assigned this pull request to a milestone

Author: knst

README.md

@@ -1,4 +1,4 @@
-Dash Core staging tree 18.0
+Dash Core staging tree
 ===========================
 
 |CI|master|develop|