Use copy() instead of byte-by-byte loop (#16222)

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**What type of PR is this?**

Optimisation

**What does this PR do? Why is it needed?**

use copy() instead of byte-by-byte loop which isn't required. 

**Which issues(s) does this PR fix?**

Fixes #

**Other notes for review**

**Acknowledgements**

- [ ] I have read
[CONTRIBUTING.md](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md).
- [ ] I have included a uniquely named [changelog fragment
file](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md#maintaining-changelogmd).
- [ ] I have added a description with sufficient context for reviewers
to understand this PR.
- [ ] I have tested that my changes work as expected and I added a
testing plan to the PR description (if applicable).

changelog/satushh-opt.md

@@ -0,0 +1,3 @@
+### Changed
+
+- Performance improvement in state (MarshalSSZTo): use copy() instead of byte-by-byte loop which isn't required.