chore: Deprecate Node.js 14.x

[duncanista]

What does this PR do?

Drops support for Node.js 14.x.

Motivation

Since it is in deprecation phase 2 by AWS.

Testing Guidelines

Automated checks.

Additional Notes

There's a note about dc-polyfill which we should check. Not done in this PR yet. https://github.com/DataDog/datadog-lambda-js/blob/039b04efca05c20d394956ad32fc4747c05097d6/src/runtime/require-tracer.ts#L1-L3

Types of Changes

• [ ] Bug fix
• [ ] New feature
• [ ] Breaking change
• [x] Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• [ ] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [ ] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [ ] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog
• [ ] This PR passes the integration tests (ask a Datadog member to run the tests)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (039b04e) 82.13% compared to head (0319c1e) 82.11%.

:exclamation: Current head 0319c1e differs from pull request most recent head 66c3ccd. Consider uploading reports for the commit 66c3ccd to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #486 +/- ## ========================================== - Coverage 82.13% 82.11% -0.03% ========================================== Files 54 54 Lines 2189 2186 -3 Branches 510 509 -1 ========================================== - Hits 1798 1795 -3 Misses 329 329 Partials 62 62 ```

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