Update fleet-agent-proxy-package-registry.asciidoc

[lucabelluccini]

I've looked in the code and credentials can be provided via xpack.fleet.registryProxyUrl: https://username:password@proxyhost:proxyport. Note for security reasons this is only allowed when using https.
On top of this, I've discovered that when using HTTPS Proxy we also ignore invalid certs by default.

https://github.com/elastic/kibana/blob/2a32ed4755c0f4852dc234b72ce5ad278a0ad593/x-pack/platform/plugins/shared/fleet/server/services/epm/registry/proxy.ts#L40C9-L40C20

[github-actions]

Warning

It looks like this PR modifies one or more .asciidoc files. These files are being migrated to Markdown, and any changes merged now will be lost. See the migration guide for details.

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @lucabelluccini? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[lucabelluccini]

This has to be back ported to at least 8.x and 7.17.x

[lucabelluccini]

FYI @kilfoyle & @kpollich

[kilfoyle]

Hi @lucabelluccini Thanks for opening this! The "Set the proxy URL" page exists only in the 8.x docs and not 7.17 (it was added in 8.10 I think). So I can backport into 8.18 and 8.17 (which is now current).

Note that we're not allowed to update earlier, non-current versions of the docs, since we have a banner at the top of those pages telling users that "No additional bug fixes or documentation updates will be released for this version.", and we don't want to false advertise. :-)

[github-actions]

Warning

It looks like this PR modifies one or more .asciidoc files. These files are being migrated to Markdown, and any changes merged now will be lost. See the migration guide for details.

[karenzone]

LGTM