Remove core-js polyfills from Kibana's server (elastic#238318)

## Summary

Removes `core-js/stable` polyfills from the Kibana server. The only
notable usage seems to have been `structuredClone`, which is now
provided by the node.js runtime natively. This also removes `core-js`
polyfills from jest configs used to test server-side code, since all of
these tests should be able to run without polyfills.

Important: `core-js` is still used to polyfill client-side code.

Resolves elastic#238657

Author: legrego

packages/kbn-eslint-plugin-i18n/jest.config.js

@@ -11,7 +11,4 @@ module.exports = {
   preset: '@kbn/test/jest_node',
   rootDir: '../..',
   roots: ['<rootDir>/packages/kbn-eslint-plugin-i18n'],
-  globals: {
-    structuredClone: {},
-  },
 };

src/platform/packages/shared/kbn-test/jest-preset.js

@@ -146,12 +146,6 @@ module.exports = {
 
   resolver: '<rootDir>/src/platform/packages/shared/kbn-test/src/jest/resolver.js',
 
-  // Workaround to "TypeError: Cannot assign to read only property 'structuredClone' of object '[object global]'"
-  // This happens when we run jest tests with --watch after node20+
-  globals: {
-    structuredClone: {},
-  },
-
   testResultsProcessor:
     '<rootDir>/src/platform/packages/shared/kbn-test/src/jest/result_processors/logging_result_processor.js',
 };

src/platform/packages/shared/kbn-test/jest_integration_node/jest-preset.js

@@ -60,7 +60,7 @@ module.exports = {
     : ['html', 'text'],
 
   snapshotSerializers: [],
-  setupFiles: ['<rootDir>/src/setup_node_env/polyfill.ts'],
+  setupFiles: [],
   haste: {
     ...preset.haste,
     throwOnModuleCollision: true,

src/platform/packages/shared/kbn-test/jest_node/jest-preset.js

@@ -17,7 +17,7 @@ delete presetClone.testEnvironment; // simply redefining as `testEnvironment: 'n
 module.exports = {
   ...presetClone,
   snapshotSerializers: [],
-  setupFiles: ['<rootDir>/src/setup_node_env/polyfill.ts'],
+  setupFiles: [],
   haste: {
     ...preset.haste,
     throwOnModuleCollision: true,

src/setup_node_env/index.js

@@ -14,6 +14,5 @@ require('./setup_env');
 require('./dns_ipv4_first');
 
 require('@kbn/babel-register').install();
-require('./polyfill');
 
 require('@kbn/security-hardening');