Broken link: regex-golang.appspot.com (Go regex tester) is dead

## Broken link

`https://regex-golang.appspot.com/assets/html/index.html` returns **404**. The
App Engine app appears to be gone.

## Where

Both files list it under a "Go regex testers" heading:

- `content/shared/influxdb-v1/flux/guides/regular-expressions.md`
- `content/shared/influxdb-v2/query-data/flux/regular-expressions.md`

Each also links [Regex101](https://regex101.com/), which is healthy (200) and
supports Go/RE2, so readers are not left without a tool.

## Why this is filed rather than fixed inline

Choosing a replacement is an editorial call, not a mechanical one — the options
differ in whether they support RE2 specifically. Candidates worth considering:

- with the Golang flavor selected (already linked)
- or the Go Playground for `regexp` snippets

Filing under `area:links` so `pr-link-check.yml` downgrades it to a warning
while the replacement is decided, instead of blocking unrelated PRs that happen
to touch these pages.

## Found via

Surfaced by #7622, which migrated the v1 copy into shared content. The link
checker only inspects changed files, so pages that had not been edited in a
while were never checked — this URL has presumably been dead for some time.

Review the two regular-expressions.md files and the link-check behavior described in pr-link-check.yml. Confirm which replacement supports Go/RE2, then update the dead link consistently in both pages. Done means the obsolete URL is removed, the chosen replacement is present, and the relevant link checks pass.

Written by the indexing model from the issue text.