Update variants for M5Dial and M5StampS3 (#10268)

Update variant for M5Dial which comes with 8MB of built-in Flash
Update variant for M5StampS3 which comes with 8MB of built-in Flash

boards.txt

@@ -22051,7 +22051,7 @@ m5stack_stamp_s3.build.flash_freq=80m
 m5stack_stamp_s3.build.flash_mode=dio
 m5stack_stamp_s3.build.boot=qio
 m5stack_stamp_s3.build.boot_freq=80m
-m5stack_stamp_s3.build.partitions=default
+m5stack_stamp_s3.build.partitions=default_8MB
 m5stack_stamp_s3.build.defines=
 m5stack_stamp_s3.build.loop_core=
 m5stack_stamp_s3.build.event_core=
@@ -22771,7 +22771,7 @@ m5stack_dial.build.flash_freq=80m
 m5stack_dial.build.flash_mode=dio
 m5stack_dial.build.boot=qio
 m5stack_dial.build.boot_freq=80m
-m5stack_dial.build.partitions=default
+m5stack_dial.build.partitions=default_8MB
 m5stack_dial.build.defines=
 m5stack_dial.build.loop_core=
 m5stack_dial.build.event_core=