Update @actions/cache and @actions/core to latest major versions

[Copilot]

Bumps @actions/cache v4→v6 and @actions/core v1→v3. Both packages dropped CommonJS in their new major versions, requiring the action scripts to be migrated to ESM.

Changes

• package.json: Updated dependency versions; added "type": "module"
• fetch.js / save.js / utils.js: Converted from CommonJS (require / module.exports) to ESM (import / export)

// Before
const core = require("@actions/core");
const cache = require("@actions/cache");
const { parseBooleanInput, buildBaseConfig } = require("./utils");

// After
import * as core from "@actions/core";
import * as cache from "@actions/cache";
import { parseBooleanInput, buildBaseConfig } from "./utils.js";

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[Copilot]

Pull request overview

Migrates this GitHub Action’s scripts to ESM to support the latest major versions of @actions/cache and @actions/core, which have dropped CommonJS.

Changes:

• Replaced CommonJS require usage with ESM import syntax in the action entry script.
• Updated local module imports to include the .js extension for ESM compatibility.

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