chore(deps): introduce a serde dependabot group (#4160)

this commit introduces a dependabot group that will manage `serde`, and
its constituent crates like `serde_json`, `serde_core`, and
`serde_derive`, in unison.

see redundant pull requests such as #4158 and
#4159 for a motivating example.

Signed-off-by: katelyn martin <[email protected]>

Author: cratelyn

.github/dependabot.yml

@@ -48,6 +48,10 @@ updates:
           - "tokio-rustls"
           - "rustls*"
           - "ring"
+      serde:
+        patterns:
+          - "serde"
+          - "serde_*"
       symbolic:
         patterns:
           - "symbolic-*"