Make fru-device produce Mfg date in ISO8601 format

Modify the time format of BOARD_MANUFACTURE_DATE in fru-device to
ISO8601 format.

Tested:
```
root@bmc:~# busctl introspect xyz.openbmc_project.FruDevice /xyz/openbmc_project/FruDevice/Minerva_CMM_EVT2
NAME                                TYPE      SIGNATURE RESULT/VALUE       FLAGS
...
xyz.openbmc_project.FruDevice       interface -         -                  -
.ADDRESS                            property  u         84                 emits-change
.BOARD_FRU_VERSION_ID               property  s         "FRU Ver 0.01"     emits-change
.BOARD_INFO_AM1                     property  s         "19-100140"        emits-change
.BOARD_INFO_AM2                     property  s         "TOD-TDT"          emits-change
.BOARD_INFO_AM3                     property  s         "D1KI6"            emits-change
.BOARD_LANGUAGE_CODE                property  s         "25"               emits-change
.BOARD_MANUFACTURER                 property  s         "Quanta"           emits-change
.BOARD_MANUFACTURE_DATE             property  s         "20240509T053800Z" emits-change
...

```

Change-Id: Ie2bcd60e4973ada81885df299fdfe132fe242828
Signed-off-by: Yi-Shum <EasonChen1@quantatw.com>
diff --git a/src/fru_utils.cpp b/src/fru_utils.cpp
index 6e6ba84..768a84d 100644
--- a/src/fru_utils.cpp
+++ b/src/fru_utils.cpp
@@ -471,7 +471,7 @@
                 // Tue Nov 20 23:08:00 2018
                 std::array<char, 32> timeString = {};
                 auto bytes = std::strftime(timeString.data(), timeString.size(),
-                                           "%Y-%m-%d - %H:%M:%S UTC", &fruTime);
+                                           "%Y%m%dT%H%M%SZ", &fruTime);
                 if (bytes == 0)
                 {
                     std::cerr << "invalid time string encountered\n";