DOCSP-47995: remove 6.0 from backport branches (#798)

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- [DOCSP-47995](https://jira.mongodb.org/browse/DOCSP-47995)
- [LATEST BUILD
LOG](https://app.netlify.com/sites/docs-ops-manager/deploys/67ec20498c00322f995fcdab)

### Self-Review Checklist

- [ ]
[Define](https://wiki.corp.mongodb.com/display/DE/Taxonomy+tagging+instructions)
taxonomy
[values](https://wiki.corp.mongodb.com/display/DE/Docs+Taxonomy) at top
of page.
- [ ] Add genre facets (tutorial or reference), as in this [example
PR](10gen/cloud-docs#5042).
- [ ] Add programmingLanguage (if necessary).
- [ ] Add meta keywords (if necessary).
- [ ] Resolve any new warnings or errors in the build.
- [ ] Proofread for spelling and grammatical errors.
- [ ] Check staging for rendering issues.
- [ ] Confirm links are working.

Author: norareidy

.backportrc.json

@@ -2,7 +2,7 @@
     "repoOwner": "10gen",
     "repoName": "docs-ops-manager",
 
-    "targetBranchChoices": ["v8.0", "v7.0", "v6.0"],
+    "targetBranchChoices": ["v8.0", "v7.0"],
 
     "editor": "code",