It appears that the partition is already at the maximum size. /dev/sda (entire disk) is 931 GiB, and that is about the same size as the extended partition.
If you want to resize just the /boot partition, you would need to decrypt it first, or use some tool from the encryption software.
EDIT:
I believe you have a fairly complicated procedure, but my experience with encrypted partitions is zero. I avoid procedures such as that, and would much rather encrypt a directory or file, if I needed it, than an entire partition; I think you can see why.
I think you need to decrypt the partition, then shrink it to make room to enlarge your /boot partition. Furthermore, you need to copy the entire partition because you need to delete the beginning of the partition in order to shrink it toward the end to make room for /boot, which is at the beginning.
There is information here, but it is complicated by mixing in instructions for both LVM and ext3; I'm not certain what you have. The links you provided are not applicable, really; your task would be very simple, although time-consuming, if you didn't have the encrypted partition. But the partitioning programs like gparted see the partition as empty, I believe.
For more information, you need to search for "how to shrink LUKS partition".