[8.x] [serverless][ESS][bugfix] Fixes outdated image on env var capture page (backport #6298)

[mergify]

Fixes #6028 by removing an outdated picture and caption from docs, as well as the image source file.

Preview: Environment variable capture

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This is an automatic backport of pull request #6298 done by Mergify.

[mergify]

Cherry-pick of b5e6d2e has failed:

On branch mergify/bp/8.x/pr-6298
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit b5e6d2ec.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   docs/cloud-native-security/environment-variable-capture.asciidoc
	deleted:    docs/cloud-native-security/images/env-var-capture.png

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/serverless/cloud-native-security/environment-variable-capture.asciidoc

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[github-actions]

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