[Merged by Bors] - chore: move Northcott from NumberTheory.Height to Order

[MichaelStollBayreuth]

Since the current module Mathlib.NumberTheory.Height.Northcott contains material that is not specific to heights (or number theory). we move it to Mathlib.Order.Northcott. This allows us to re-use the old module location to add a height-specific Northcott instance in a follow-up PR.

---

[github-actions]

PR summary f40a8e8c6e

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.NumberTheory.Height.Northcott	-592
Mathlib.Order.Northcott	592

---

Declarations diff

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

## summary with just the declaration names:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

---

No changes to strong technical debt.

No changes to weak technical debt.

---

note: file Mathlib/NumberTheory/Height/Northcott.lean` was renamed to `Mathlib/Order/Northcott.lean without a module deprecation
Please create a follow-up pull request adding one. Thanks!

[faenuccio]

Thanks!

bors d=tb65536

[mathlib-bors]

✌️ tb65536 can now approve this pull request. To approve and merge a pull request, simply reply with bors r+. More detailed instructions are available here.

[tb65536]

bors r+

(usually we would add a deprecation, but this will get resolved in a few days hopefully)

[mathlib-bors]

Pull request successfully merged into master.

Build succeeded:

• ci (staging) / Post-CI job
• ci (staging) / Post-Build Step
• ci (staging) / Lint style
• ci (staging) / Build