Summary:
This change improves error messages while validating previw GFlags
as follows:
  - Adds the serverType indicating if the error is for master/tserver
  - Adds the list of preview flags which have been set but not
    included in the allowed_preview_flags_csv
  - Fixes our API examples for upgrade gflags

Test Plan:
Ran the following tests
  - Set preview flags for master and keep allowed_preview_flags_csv empty
    and ensure that the right error message shows up
  - Set preview flags for tserver abd keep allowed_preview_flags_csv_empty
    and ensure that the right error message shows up
  - Set multiple preview flags for master and add only a subset to
    allowed_preview_flags_csv and ensure that the right error message shows up
  - Set multiple preview flags for tserver and add only a subset to
    allowed_preview_flags_csv and ensure that the right error message shows up
  - Set preview flags and allowed_preview_flags_csv for both master and
    tserver and ensure that the API is successfully submitted

Reviewers: vbansal, daniel, anijhawan

Reviewed By: vbansal, anijhawan

Subscribers: svc_phabricator, anijhawan, yugaware

Differential Revision: https://phorge.dev.yugabyte.com/D40017