Update packages, migrate to ESM

[Samirat]

Description

• Update cache to the most recent version published from toolkit (v6)
• Migrate to ESM (required in order to consume new packages)

Motivation and Context

Related to https://github.com/github/c2c-actions-support/issues/6176 and in general this package needed more work to keep it from falling farther out of date.
The ESM migration is necessary in order to consume any new versions of the cache package

How Has This Been Tested?

• Local Tests

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (add or update README or docs)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[Copilot]

Pull request overview

This PR updates the action to the latest @actions/* toolkit majors (cache v6) and attempts to migrate the repository/tooling to ESM, including updating documentation references and adjusting the Jest test setup for ESM.

Changes:

• Bump action/toolkit dependencies to @actions/cache@^6.0.1 and related @actions/* majors; update release notes and docs to reference actions/cache@v6.
• Switch the package to ESM ("type": "module") and update TS compiler settings for newer Node/ES targets.
• Migrate Jest configuration/tests to an ESM-compatible setup (jest.config.ts, unstable_mockModule, ESM transforms).

Show a summary per file

File	Description
tsconfig.json	Updates TS target/module settings for ESM + modern Node, adjusts excludes.
tips-and-workarounds.md	Updates usage examples to actions/cache@v6.
RELEASES.md	Adds v6.0.0 changelog entry.
README.md	Updates docs to reference actions/cache@v6 and notes v6 changes.
package.json	Bumps version/deps, sets ESM package type, updates Jest invocation.
jest.config.ts	Adds new TS-based Jest config for ESM + ts-jest.
jest.config.js	Removes previous CJS Jest config.
examples.md	Updates usage examples to actions/cache@v6.
caching-strategies.md	Updates usage examples to actions/cache@v6 / restore/save @v6.
tests/stateProvider.test.ts	Migrates tests to ESM-style mocking/imports.
tests/saveOnly.test.ts	Migrates tests to ESM-style mocking/imports.
tests/saveImpl.test.ts	Migrates tests to ESM-style mocking/imports; adjusts warning expectations.
tests/save.test.ts	Migrates tests to ESM-style mocking/imports.
tests/restoreOnly.test.ts	Migrates tests to ESM-style mocking/imports.
tests/restoreImpl.test.ts	Migrates tests to ESM-style mocking/imports; adjusts error simulation.
tests/restore.test.ts	Migrates tests to ESM-style mocking/imports.
tests/actionUtils.test.ts	Migrates tests to ESM-style mocking/imports; simplifies assertions.

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• Files reviewed: 15/18 changed files
• Comments generated: 6