chore: Golf proofs in FDeriv/Symmetric and DominatedConvergence

[yuanyi-350]

This PR shortens two proofs by using existing lemmas and more standard constructions.

• In Analysis/Calculus/FDeriv/Symmetric, it replaces a manual construction of the restricted second derivative with bilinearRestrictScalars, and streamlines the ContDiffWithinAt.isSymmSndFDerivWithinAt argument by passing through the corresponding ContDiffAt result.
• In MeasureTheory/Integral/Lebesgue/DominatedConvergence, it replaces the recursive max construction with a finite iSup over Set.Iic n, which simplifies the measurability, monotonicity, and integral comparison arguments.

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[github-actions]

PR summary 56c5f346fa

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

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

The doc-module for scripts/pr_summary/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/reporting/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).

[mathlib-bors]

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