[8.x] [Fleet & Agent docs] Change 'beats' namespace to 'elastic-agent' (backport #1315)

[mergify]

Elastic Agent Docker images are currently available under both a beats and elastic-agent namespace. For 9.0 only the latter will be available, so in preparation for that this will update the current (8.15).

Closes: #1314

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This is an automatic backport of pull request #1315 done by Mergify.

[github-actions]

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