chore: deprecate sub_lt_zero

[Parcly-Taxel]

It is a strictly longer alias of sub_neg and therefore wholly redundant.

[github-actions]

PR summary 3212c3f00c

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff (regex)

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.

Declarations diff (Lean)

✅ Lean-aware diff — post-build, computed from the Lean environment (commit 3212c3f).

• +0 new declarations
• −0 removed declarations

No declaration differences.

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No changes to strong technical debt.
No changes to weak technical debt.

Current commit 3212c3f00c
Reference commit b1007d8abf

This script lives in the mathlib-ci repository. To run it locally, from your mathlib4 directory:

git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci
../mathlib-ci/scripts/reporting/technical-debt-metrics.py pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

[themathqueen]

I don't think we should deprecate this alias. I don't think it being a longer name means we should deprecate it. sub_neg can sound like a - -b, so having a more descriptive alias is helpful.