chore: upgrade coder/serpent to allow more readable durations (#20886)

https://github.com/coder/serpent/pull/28 added this capability.

https://github.com/coder/serpent/compare/v0.11.0...v0.12.0

---------

Signed-off-by: Danny Kopping <danny@coder.com>

cli/testdata/coder_server_--help.golden

@@ -106,7 +106,7 @@ AIBRIDGE OPTIONS:
           See
           https://docs.claude.com/en/docs/claude-code/settings#environment-variables.
 
-      --aibridge-retention duration, $CODER_AIBRIDGE_RETENTION (default: 1440h)
+      --aibridge-retention duration, $CODER_AIBRIDGE_RETENTION (default: 60d)
           Length of time to retain data such as interceptions and all related
           records (token, prompt, tool use).
 

cli/testdata/server-config.yaml.golden

@@ -753,5 +753,5 @@ aibridge:
   inject_coder_mcp_tools: false
   # Length of time to retain data such as interceptions and all related records
   # (token, prompt, tool use).
-  # (default: 1440h, type: duration)
+  # (default: 60d, type: duration)
   retention: 1440h0m0s