product-selector: v1 alt_link key collides between InfluxDB OSS v1 and InfluxDB Cloud 1

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## Problem

`layouts/partials/topnav/product-selector.html` uses the alt_link key `v1` for both **InfluxDB OSS v1** (line 16) and **InfluxDB Cloud 1** (line 31):

```
{{ $influxdbOSSv1 := dict "influxdb/v1" (slice "InfluxDB OSS v1" "v1") }}
...
{{ $influxdbCloud1 := dict "influxcloud/v1" (slice $.Site.Data.products.influxdb_cloud1.name "v1") }}
```

When a page's frontmatter declares `alt_links: { v1: /some/path/ }`, the `productLink` template looks up the key and applies the same URL to both the OSS v1 slot and the Cloud 1 slot of the product selector dropdown — regardless of which product the link was intended for.

## Repro

1. Visit a page whose frontmatter sets `alt_links: { v1: /influxdb/v1/some-path/ }` (for example, any v1 release-notes or API page).
2. Open the product selector.
3. Both **InfluxDB OSS v1** and **InfluxDB Cloud 1** entries point to the same OSS v1 URL.

## Fix options

- Give Cloud 1 a distinct key, e.g. `cloud_v1` (parallel with `enterprise_v1`).
- Or: drop alt_link support for `influxcloud/v1` entirely and always use its special-link path (it already has a `$specialLink` branch in `productLink`).

Either way, any existing page-level `v1:` alt_links will still resolve correctly for OSS v1, and Cloud 1 will route through an unambiguous key or the special-link branch.

## Context

Found while auditing alt_link keys during #6622 (feat-api-uplift). Separate from that PR's fix for the `enterprise-v1` → `enterprise_v1` mismatch in `data/api_products.yml`.

Recap:

- Fixed in #6622 : data/api_products.yml's enterprise-v1 → enterprise_v1 key (commit a08dde3c1). That was the API pages' alt_links mapping to
layouts/partials/topnav/product-selector.html's enterprise_v1 key for enterprise_influxdb/v1.

Start in layouts/partials/topnav/product-selector.html and inspect the alt_link keys for the InfluxDB OSS v1 and Cloud 1 entries, including the productLink lookup and special-link branch. Choose one of the stated fixes with the maintainer, then reproduce the page-level v1 alt_link case and verify the product selector no longer sends both entries to the same URL.

Written by the indexing model from the issue text.