Tell Dependabot to ignore higher versions of chai and pixelmatch (#7159)

* Don't try to update chai higher than 4.x.x.

Updating chai seems to cause the ts checks to fail. Based on [this
comment](chaijs/chai#1568 (comment))
and others in an issue thread in the Chai project repo, version 5.0.0
and higher of chai requires a ceratin type of module use. Perhaps this
is the reason why the tests fail for us -- I can't tell without
looking more closely. Right now, this is not an essential update to
do, and we need to stop Dependabot trying to update this every time it
runs.

* Don't try to update pixelmatch higher than 5.3.0

Author: mhucka

.github/dependabot.yml

@@ -45,6 +45,11 @@ updates:
     schedule:
       interval: "monthly"
     versioning-strategy: "widen"
+    ignore:
+      - dependency-name: "chai"
+        versions: [">= 5.0.0"]
+      - dependency-name: "pixelmatch"
+        versions: ["> 5.3.0"]
     groups:
       # Group non-security version minor & patch updates into one PR.
       # Security and major update versions will be done as individual PRs.