docs: update Ubuntu compatibility for version 4.2.0 (Resolves #9640)

[yasithsamaranayake]

Purpose

Update the product compatibilty documentation to remove references to unsupported Ubuntu versions (18.04, 20.04) and ensure only Ubuntu 22.04 is listed.

Resolves #9640

Goals

Align installation documentation with Ubuntu's official release lifecycle.
Ensure that users follow up-to-date guidance when installing WSO2 API Manager 4.2.0.

Approach

Edited the product compatibility page for version 4.2.0:

'en/docs/install-and-setup/setup/reference/product-compatibility.md'

Removed Ubuntu 18.04 and 20.04 from the compatibility matrix.
Verified Ubuntu 22.04 as the installed version.

User stories

As a user installing/using API Manager 4.2.0, I want documentation that lists only supported OS versions, so I don't waste time with outdated releases.

Release note

Updated the product compatibility documentation for WSO2 API Manager 4.2.0 to reflect supported Ubuntu version (22.04 only).

Documentation

This PR updates the 4.2.0 compatibility page:

https://apim.docs.wso2.com/en/4.2.0/install-and-setup/setup/reference/product-compatibility/

Training

N/A - no training content affected.

Certification

N/A - no certification impact.

Marketing

N/A - no marketing content affected.

Automation tests

• Unit tests

  N/A (documentation-only change)

• Integration tests

  N/A

Security checks

• Followed secure coding standards in http://wso2.com/technical-reports/wso2-secure-engineering-guidelines? yes
• Ran FindSecurityBugs plugin and verified report? N/A
• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets? yes

Samples

N/A

Related PRs

PR #9663 raised for same change in version 3.2.0
PR #9664 raised for same change in version 4.0.0
PR #9665 raised for same change in version 4.1.0
A separate PR will be raised for version 4.3.0 to fully resolve #9640

Migrations

N/A

Test environment

Documentation-only change - no runtime environment required

Learning

Referenced Ubuntu EOL documentation:
https://ubuntu.com/blog/ubuntu-20-04-lts-end-of-life-standard-support-is-coming-to-an-end-heres-how-to-prepare
https://ubuntu.com/blog/ubuntu-18-04-eol-for-devices