docs: use word "UNIX timestamp" for consistency

[kenjis]

Description

• use word "UNIX timestamp" for consistency

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[paulbalandan]

Where is the guidance for using UNIX? I'm also using "Unix".

[kenjis]

Search the user guide. We use "UNIX timestamp" in other places. There is only one "Unix timestamp".

Maybe "Unix" is appropriate.

Unix (/ˈjuːnɪks/, YOO-niks; trademarked as UNIX)
https://en.wikipedia.org/wiki/Unix

[kenjis]

Can someone approve?