chore(Polynomial/Div): deprecate noncomputable Decidable non-instance

[plp127]

Deprecate decidableDvdMonic, which is a noncomputable Decidable non-instance and so inferior to Classical.dec. This is a leftover from before leanprover-community/mathlib3#1391, in which polynomials were made to use classical logic.

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[github-actions]

PR summary 789108e1c8

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

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

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh 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).