ci(release-please): fix footer not appearing on release PRs

Release-please defaults `separatePullRequests` to `true` for
single-package configs, which skips the Merge plugin that applies
the configured `pull-request-footer`. Setting `separate-pull-requests`
to `false` explicitly forces the Merge plugin to run.

See: https://github.com/googleapis/release-please/issues (upstream bug)
Signed-off-by: Gregor Zeitlinger <gregor.zeitlinger@grafana.com>

Author: zeitlinger

.github/config/release-please-config.json

@@ -1,5 +1,6 @@
 {
   "$schema": "https://raw.githubusercontent.com/googleapis/release-please/main/schemas/config.json",
+  "separate-pull-requests": false,
   "pull-request-footer": "> [!IMPORTANT]\n> Close and reopen this PR to trigger CI checks.",
   "packages": {
     ".": {