chore(ModelTheory): fix hypo of realize_liftAt

[zhuyizheng]

changes the hypo of realize_liftAt from the incorrect (hmn : m + n' ≤ n + 1) to the correct (hmn : m ≤ n)

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[github-actions]

PR summary c033d3e388

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

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.

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No changes to strong technical debt.

No changes to weak technical debt.

[plp127]

this seems to be not strictly stronger?

[zhuyizheng]

this seems to be not strictly stronger?

The strongest form is m ≤ n ∨ n' = 0, but

1. mathematically, liftAt makes sense only when m ≤ n;
2. if anyone ever needs n' = 0, the left hand side (φ.liftAt n' m).Realize v xs should immediately be simplified to φ.Realize v xs. The simplification from φ.liftAt 0 m to φ deserves a separate lemma with a @[simp] attribute, in another PR.