style: fix linting issues in store module (#15724)

## Description

- Return an exported type rather than an unexported type, from an exported function
- Assign error values to _
- Add reason to nolint directive
- Remove ineffectual assignment
- Remove checked cast to same type as value

---

### Author Checklist

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- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed all author checklist items have been addressed
- [ ] confirmed that this PR does not change production code

store/cachekv/internal/mergeiterator.go

@@ -24,7 +24,7 @@ type cacheMergeIterator struct {
 
 var _ types.Iterator = (*cacheMergeIterator)(nil)
 
-func NewCacheMergeIterator(parent, cache types.Iterator, ascending bool) *cacheMergeIterator { //nolint:revive
+func NewCacheMergeIterator(parent, cache types.Iterator, ascending bool) types.Iterator {
 	iter := &cacheMergeIterator{
 		parent:    parent,
 		cache:     cache,