ci(workflows): remove conflict markers from update-version-tags.yml

`.github/workflows/update-version-tags.yml` has carried literal git
conflict markers in its YAML header since the Mergify auto-backport
in #1988 (cherry-pick of #1984 onto circinus) — the cherry-pick
conflicted, but Mergify's `ignore_conflicts: true` default at the
time committed the markers verbatim into the file. (The central
Mergify config switched to `ignore_conflicts: false` a few days
later as a direct response to this class of incident; #1988 had
already merged.)

Result: GitHub's YAML parser fails on `<<<<<<<` / `=======` /
`>>>>>>>` and the workflow can't schedule any job. Every push to
circinus touching `docs/**` or `context7.json` since #1988 merged
reported "this run likely failed because of a workflow file issue"
with 0 jobs. The downstream `context7-refresh.yml` (which uses
`workflow_run` to react to this workflow's name) also never fired,
so Context7's `1.5` index has been getting stale on every doc push.

Fix: replace the file with rolling's verbatim content. The
post-conflict-marker content on circinus was already byte-identical
to rolling; only the marker block + the obsolete comment block
above it differ. Confirmed with
`diff <(git show origin/rolling:.../update-version-tags.yml) <(...)`
returning empty after stripping markers.

After this merges:
- `Update version tags` runs on every docs-affecting push to
  circinus, moves the `1.5` tag to HEAD.
- `Context7 refresh` fires via `workflow_run` and refreshes the
  `1.5` index.

🤖 Generated by [robots](https://vyos.io)

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