test(coverage): increase fail_under to 81 (#1361)

**Issue number:** N/A

## Summary

The coverage was improved by recent PRs so changing the `fail_under` in
`.coveragerc` to reflect it.

### Changes

> Please provide a summary of what's being changed

N/A

### User experience

> Please describe what the user experience looks like before and after
this change

N/A

## Checklist

If your change doesn't seem to apply, please leave them unchecked.

* [x] I have performed a self-review of this change
* [x] Changes have been tested
* [ ] Changes are documented
* [x] PR title follows [conventional commit
semantics](https://www.conventionalcommits.org/en/v1.0.0/)

.coveragerc

@@ -6,4 +6,4 @@ omit =
     splunk_add_on_ucc_framework/templates/input.module-template
 
 [report]
-fail_under = 78.5
+fail_under = 81