ci: bump paradedb/actions to v8 for the upstream sync (#42)

## What

Bump the three `paradedb/actions` references used by the upstream sync
from `v7` to `v8`:

- `.github/workflows/sync-upstream-rebase.yml` —
`reusable-rebase.yml@v7` → `@v8`
- `.github/workflows/sync-promote-branch.yml` —
`reusable-promote.yml@v7` → `@v8`
- `scripts/sync-upstream.sh` — `.../actions/v7/scripts/sync-core.sh` →
`.../v8/...`

## Why

The scheduled **Upstream Rebase** was failing when the runner hit `No
space left on device`: the reusable workflow checked out with
`fetch-depth: 0` and then did a bare `git fetch`, pulling *every* branch
and its unrelated history and filling the runner disk. `v8` fixes this
with `fetch-depth: 1` plus a `fetch_branch` helper that fetches only the
target and upstream `main` branches (paradedb/actions#8). It also adds a
`range-diff` subcommand (paradedb/actions#11).

## Note

All three references must move together — `sync-upstream.sh` downloads
`sync-core.sh` from the version path, so bumping just the workflow refs
would still execute the old core script. The `v8` tag is already
published (signed/verified) on `paradedb/actions`.

https://claude.ai/code/session_016i7usbUvUuBwHsv13uaa4S

Author: philippemnoel

.github/workflows/sync-promote-branch.yml

@@ -19,7 +19,7 @@ concurrency:
 
 jobs:
   call-promote:
-    uses: paradedb/actions/.github/workflows/reusable-promote.yml@v7
+    uses: paradedb/actions/.github/workflows/reusable-promote.yml@v8
     with:
       branch_name: ${{ inputs.branch_name }}
       approvers: "philippemnoel,rebasedming,stuhood,mdashti"

.github/workflows/sync-upstream-rebase.yml

@@ -17,7 +17,7 @@ concurrency:
 
 jobs:
   call-rebase:
-    uses: paradedb/actions/.github/workflows/reusable-rebase.yml@v7
+    uses: paradedb/actions/.github/workflows/reusable-rebase.yml@v8
     with:
       github_app_client_id: ${{ vars.PARADEDB_GITHUB_APP_CLIENT_ID }}
       approvers: "philippemnoel,rebasedming,stuhood,mdashti"

scripts/sync-upstream.sh

@@ -14,7 +14,7 @@ export UPSTREAM_BRANCH="main"
 
 # 2. Define the URL to the centralized script
 # Using the raw content URL from the central repository
-CORE_SCRIPT_URL="https://raw.githubusercontent.com/paradedb/actions/v7/scripts/sync-core.sh"
+CORE_SCRIPT_URL="https://raw.githubusercontent.com/paradedb/actions/v8/scripts/sync-core.sh"
 
 # 3. Download and source the core logic as an API
 TMP_SCRIPT=$(mktemp)