Version 8.2 does not work with with Nodejs 12 and below due to the new version of undici

[bugb]

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🐛 Bug Report

• Since undici version 5.28.0, they will not support Nodejs 12 and below. It has been confirmed Undici does not work with Nodejs 12 and below nodejs/undici#2469 (comment) or you can check the diff from the version v5.27 latest to the version 5.28 here where you can see a lot of ?? (optional chainning) and ?. (nullish coalescing) synxtax those are not availble in Nodejs 12 and below.

• This problem happens due to the line below:

undici will be resolved to the version under 6.0, due to npm follows semver spec. Or you can check the online version of npm semver calculation here.

To Reproduce

Steps to reproduce the behavior:

• Use elastic-transport-js version 8.2 in Nodejs 12 and below

Expected behavior

• elastic-transport-js should work with Nodejs 12 because it is in high demand :)
  (Source)
  

Your Environment

• node version: 12 and below
• @elastic/elasticsearch version: 8.2
• os: Mac, Windows, Linux
• any other relevant information

Additional information

• I have created a PR fix: undici compatible with Nodejs 12 and below #75, feel free to review it.
• When run the coverage test for both version 8.2 and 8.2.fix, the both coverage reports are indentical!
  

[JoshMock]

As noted in the client's documentation, the Elasticsearch Node.js client and supporting libraries like this one will drop support for Node.js versions that have reached end of life between minor versions. Node.js v12 reached EOL in April of 2022, so we no longer support it as of client version 8.4.0. Support for v14 and v16 will end when I ship 8.11.0 of the client in the next few days.