use rails7 option for brakeman

flag=none

Test Plan:
- specs pass

Change-Id: I081f5a545b0f0d21f0206bef0ef5ec61f71745c8
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/311974
Reviewed-by: Isaac Moore <isaac.moore@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: Spencer Olson <solson@instructure.com>
Product-Review: Spencer Olson <solson@instructure.com>

script/brakeman

@@ -7,7 +7,7 @@ linter_options = {
   linter_name: "brakeman",
   file_regex: /\.rb$/,
   format: "brakeman",
-  command: "bundle exec brakeman --rails6 --quiet --format json --confidence-level 2 -i config/brakeman.ignore",
+  command: "bundle exec brakeman --rails7 --quiet --format json --confidence-level 2 -i config/brakeman.ignore",
   append_files_to_command: false,
   severe_anywhere: false
 }