CI: Only get security updates (#56)

#### Problem

Dependabot updates all versions of packages, which is less flexible for
end users. This made sense for agave, which is focused on providing a
binary, but libraries are more useful when dependencies are relaxed.

#### Summary of changes

Change the open pull request number to 0 to only enable security
updates, as documented at
[Dependabot's documentation](https://docs.github.com/en/code-security/dependabot/dependabot-security-updates/configuring-dependabot-security-updates#overriding-the-default-behavior-with-a-configuration-file)

Fixes #55

Author: joncinque

.github/dependabot.yml

@@ -11,4 +11,4 @@ updates:
     interval: daily
     time: "08:00"
     timezone: UTC
-  open-pull-requests-limit: 6
+  open-pull-requests-limit: 0