remove: remove stats/base/nanmean

This commit removes `@stdlib/stats/base/nanmean` in favor of `@stdlib/stats/strided/nanmean`.

BREAKING CHANGE: remove `stats/base/nanmean`

To migrate, users should update their require/import paths to use
`@stdlib/stats/strided/nanmean`, which provides the same API and implementation.

Ref: #4797

---
type: pre_commit_static_analysis_report
description: Results of running static analysis checks when committing changes.
report:
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---

Author: gururaj1512