MINOR: [R] Add checking C++ updates to release checklist (#48273)

### Rationale for this change

We don't always notice bindings to C++ changes we after releases

### What changes are included in this PR?

Update release checklist to include checking C++ for updates

### Are these changes tested?

No

### Are there any user-facing changes?

No

Authored-by: Nic Crane <[email protected]>
Signed-off-by: Nic Crane <[email protected]>

Author: thisisnic

r/PACKAGING.md

@@ -87,3 +87,4 @@ _This step must be done by the current package maintainer._
 - [ ] Wait for CRAN-hosted binaries on the [CRAN package page](https://cran.r-project.org/package=arrow) to reflect the new version.
 - [ ] Post already-prepared content to social media.
   - Use Bryce's [script](https://gist.githubusercontent.com/amoeba/4e26c064d1a0d0227cd8c2260cf0072a/raw/bc0d983152bdde4820de9074d4caee9986624bc5/new_contributors.R) for contributor calculation.
+- [ ] Check C++ updates for this release and create issues for any items which need bindings in R