[8.18] [Internal][8.18 & 8.19][Security]: Fix bugs in alert suppression docs and apply improvements (backport #7083)

[mergify]

Fixes elastic/docs-content#3305 by doing the following:

• Fix the outdated images in the 8.18 and 8.19 alert suppression docs (alert-suppression-options.png and suppressed-alerts-details.png)
• Apply the content improvements that were also added in [Security][9.2 & Serverless]: New advanced setting that controls suppression window behavior docs-content#2735.

Preview

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This is an automatic backport of pull request #7083 done by Mergify.

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