perf: Speedup Dec.Sqrt() (backport #16141)

[mergify]

This is an automatic backport of pull request #16141 done by Mergify.
Cherry-pick of fb8ff07 has failed:

On branch mergify/bp/release/v0.47.x/pr-16141
Your branch is up to date with 'origin/release/v0.47.x'.

You are currently cherry-picking commit fb8ff071c.
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Changes to be committed:
	modified:   math/dec.go
	modified:   math/dec_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   math/CHANGELOG.md

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[julienrbrt]

This does not need to be backported, we simply need to tag a new math.