chore: drop Node.js 18 support

[dreamorosi]

Summary

Changes

Please provide a summary of what's being changed

This PR drops support for Node.js 18 by:

• increasing the minimum engine version in the main package.json & package-lock.json
• removing the 18.x from GitHub Issue templates
• removing 18 from all matrixes in GitHub Actions
• removing Node.js 18 from the badge in the main README.md file
• removing test:e2e:nodejs18x commands from all package.json files
• removing the NODEJS_18_X runtime and increasing the minimum runtime to NODEJS_20_X in all Lambda layer configurations
• setting the compile target to ES2023 (aka Node.js 20) in the main tsconfig.json file
• removing Middy.js 4.x from dev dependencies and tests, since 5.x (new minimum supported version by Powertools for AWS) supports only Node.js 20 and above

Please add the issue number below, if no issue is present the PR might get blocked and not be reviewed

Issue number: closes #3562

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[dreamorosi]

End to end tests on this branch are green: https://github.com/aws-powertools/powertools-lambda-typescript/actions/runs/16656498496

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud