Increment package versions to 2.9.1 following today's release. (#19090)

A release was published today:
* https://pypi.org/project/iree-base-compiler/2.9.0/
* https://pypi.org/project/iree-base-runtime/2.9.0/
* https://github.com/iree-org/iree/releases/tag/iree-2.9.0rc20241108

We're still figuring out the procedure for these new version updates,
but https://github.com/iree-org/iree/issues/18938 does at least say:

> After a release, either Y or Z MUST be increased to ensure precedence
of nightly builds.

We can decide next week if we want to start by updating the minor
version instead of the patch version that I updated here.

compiler/version.json

diff --git a/compiler/version.json b/compiler/version.json
index ca3c0ed..794a2de 100644
--- a/compiler/version.json
+++ b/compiler/version.json
@@ -1,3 +1,3 @@
 {
-  "package-version": "2.9.0.dev"
+  "package-version": "2.9.1.dev"
 }

runtime/version.json

diff --git a/runtime/version.json b/runtime/version.json
index ca3c0ed..794a2de 100644
--- a/runtime/version.json
+++ b/runtime/version.json
@@ -1,3 +1,3 @@
 {
-  "package-version": "2.9.0.dev"
+  "package-version": "2.9.1.dev"
 }