refactor: rename Cardinal.ord_eq → Cardinal.exists_ord_eq

[vihdzp]

As suggested in #36928.

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• depends on: [Merged by Bors] - feat: bundled version of Cardinal.ord_eq #36928

[github-actions]

PR summary 47aa862678

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

+ exists_ord_eq

You can run this locally as follows

## 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-dependent-issues]

This PR/issue depends on:

• [Merged by Bors] - feat: bundled version of Cardinal.ord_eq #36928
  By Dependent Issues (🤖). Happy coding!