Automatically create PRs to bump versions in ndc-postgres (#594)

<!-- The PR description should answer 2 (maybe 3) important questions:
-->

### What

<!-- What is this PR trying to accomplish (and why, if it's not
obvious)? -->

<!-- Consider: do we need to add a changelog entry? -->

This PR adds a `bump-version.yaml` workflow that can be manually
triggered in the `Actions` tab. When triggered and given a new version
of `ndc-postgres`, a PR will be generated that bumps `ndc-postgres` to
that version. In practice, this means:

* Updating the `Cargo.toml` file and associated lock file.
* Moving everything in `changelog.md` from `Unreleased` to a section
with the appropriate version title, and creating a new, blank
`Unreleased` section.

### How

<!-- How is it trying to accomplish it (what are the implementation
steps)? -->
GitHub actions, including a fancy new action we found to create pull
requests against the repo.

.github/workflows/bump-version.yaml

@@ -0,0 +1,33 @@
+on:
+  workflow_dispatch:
+    inputs:
+      new_version:
+        description: "What we want the new version to be"
+        required: true
+
+name: Bump version
+
+jobs:
+  bump_version_pr:
+    name: Create version bump PR
+    runs-on: ubuntu-latest
+    env:
+      CARGO_NET_GIT_FETCH_WITH_CLI: "true"
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install `just`
+        uses: extractions/setup-just@v2
+
+      - uses: Swatinem/rust-cache@v2
+        with:
+          shared-key: "build" # share the cache across jobs
+          save-if: false
+
+      - run: ./scripts/new-version.sh ${{ github.event.inputs.new_version }}
+
+      # TODO: auto-merge when we feel confident.
+      - uses: peter-evans/create-pull-request@v6
+        with:
+          title: Bump version to ${{ github.event.inputs.new_version }}
+          body: This bumps the version in `Cargo.toml` and updates `changelog.md`.