Switch to pull request list API instead of search (#393)

* Switch to pull request list API instead of search

To reduce the amount of rate-limit that we use,
switch to a cheaper API for determining whether
an existing PR exists for a given branch.

* Ensure PR remains open when updating description

* Update .changeset/clean-bees-count.md

Co-authored-by: willo-icon <161356817+willo-icon@users.noreply.github.com>

---------

Co-authored-by: Mateusz Burzyński <mateuszburzynski@gmail.com>
Co-authored-by: willo-icon <161356817+willo-icon@users.noreply.github.com>

.changeset/clean-bees-count.md

@@ -0,0 +1,5 @@
+---
+"@changesets/action": patch
+---
+
+Ensure the PR remains open when updated

.changeset/green-eels-appear.md

@@ -0,0 +1,5 @@
+---
+"@changesets/action": patch
+---
+
+Switch to cheaper API for querying existing PRs