feat: add desktop 7.1

[DeepDiver1975]

This PR enables the documentation for desktop 7.1.

Following the Create a New Version Branch process (Step 5) in
docs-client-desktop:docs/new-version-branch.md, and matching the precedent of
#5104 (iOS 12.7) and #5099 (android 4.8).

Changes

• site.yml: add the 7.1 branch of docs-client-desktop to the build.
• global-attributes.yml: latest-desktop-version: 7.1, previous-desktop-version: 6.0.

Note: 5.3 intentionally kept

Unlike the usual "latest two named versions" convention (and unlike #5104/#5099), the
oldest version 5.3 is NOT dropped — it is still widely used. The desktop docs therefore
publish four versions: next + 7.1 + 6.0 + 5.3. No branch is archived.

⚠️ Merge prerequisites / timing

• Depends on the 7.1 branch existing in docs-client-desktop (Steps 1–4 of the process).
  The build will fail until that branch is pushed and protected.
• Per the process doc, merge close before the 7.1 product tag is published, otherwise the
  webserver latest pointer (driven by the product's latest release tag) will 404.

🤖 Generated with Claude Code

[DeepDiver1975]

🤖 Automated review by Claude Code review agent.

Overview

This PR enables documentation publishing for desktop client 7.1. It is a small, surgical change (2 files, +3/-2) following the established Create a New Version Branch process and the precedent set by #5104 (iOS 12.7) and #5099 (android 4.8):

• site.yml: adds the 7.1 branch of docs-client-desktop to the Antora content sources.
• global-attributes.yml: bumps latest-desktop-version 6.0 → 7.1 and previous-desktop-version 5.3 → 6.0.

I verified the change against the live repos. It is correct and internally consistent. Approving in spirit; only a timing/merge-ordering caveat remains.

Code quality / style

• YAML formatting matches the surrounding conventions exactly (2-space indent, single-quoted version strings, new branch inserted at the top of the list in descending order — same pattern as the iOS/android precedents).
• Attribute bump is in lockstep and consistent: latest=7.1, previous=6.0, and both of those branches are present in site.yml. No dangling or mismatched version references introduced.
• No AsciiDoc/xref changes in this diff (no new pages are added here — page content lives in the 7.1 branch of docs-client-desktop), so there is no broken-link surface in this PR itself.

Specific suggestions

• Merge prerequisite is already satisfied. The PR description warns "the build will fail until the 7.1 branch is pushed and protected." I confirmed the 7.1 branch already exists in owncloud/docs-client-desktop and its antora.yml declares name: desktop / version: '7.1'. The master branch declares version: 'next' (prerelease). So the published set will be exactly next + 7.1 + 6.0 + 5.3 as described — the build should aggregate cleanly. You may want to update the description to note the branch now exists.
• Keeping 5.3 is a deliberate, documented divergence from the "latest two named versions" convention used in #5104/#5099. This is clearly called out in the description and is the right call if 5.3 is still widely deployed. No action needed; just flagging that it is intentional so a future reviewer doesn't "fix" it by dropping 5.3.

Potential issues / risks

• Timing risk (the real one). Per the process doc, this must be merged close before the 7.1 product release tag is published, because the webserver latest pointer is driven by the product's latest release tag — merging too early (before the tag exists) risks a 404 on the latest desktop docs URL. This is correctly noted in the PR description; the only "risk" is operational ordering, not a defect in the diff. Coordinate the merge with the 7.1 product tag.
• No next/master regression. site.yml keeps master (which maps to the next prerelease docs), so the prerelease channel is unaffected.

Verdict

LGTM. The diff is minimal, follows precedent, and all version references are consistent. The only blocker is non-code: merge timing relative to the 7.1 product tag. No changes requested.