As you intimate in your question, the Luke passage concerns forgiveness. The apostles found it hard to believe it possible they could forgive a brother who sins against them seven times in a single day, even if he repents each and every time. After Jesus told them they must nevertheless forgive the sinning brother, they said to Jesus,
"Lord, increase our faith" (17:5).
Jesus then teaches them it's not the size of faith that matters. Even small faith can accomplish great things. Why, a 30-foot tall mulberry tree, which by nature has an elaborate and extensive root structure, could be uprooted and planted in the sea by a simple faith-based command.
By implication, Jesus was saying that extending forgiveness is not a matter of faith at all. Rather, it is a simple act of obedience to their Lord.
Jesus then goes on to expand His teaching by drawing an analogy involving a slave and a master. Allow me to paraphrase Jesus' words:
When a slave comes in after a hard day's work, be it plowing or tending sheep, does the master say, "Hey, my friend, seems like you've had a rough day. Sit down, relax, put your feet up, and have a nosh"? Of course not! The master has every right to tell the slave to prepare his master's meal, change into some clean clothes, and then come and serve the food to him. When the master is full and satisfied, then--and only then--is the slave free to get a bite to eat.
Moreover, the master isn't obligated to say "Oh, thank you ever so much, my man, for serving me so unselfishly!" Why? Because the slave was merely doing his duty. In the same way, by forgiving a brother who sins against you, you are simply performing your duty to me, and I, Jesus, your lord and master am telling you to forgive.
In other words, the import of this passage has nothing to do with miracles. Frankly, uprooting a tree and planting it in the ocean is not a miracle; it's more like a stunt. Jesus is clearly using hyperbole to get His point across about the importance of forgiveness.
". . . and forgive us our debts as we forgive our debtors . . ."
as Jesus taught us to pray.
To summarize, the Luke passage is not about miracles, although an argument could be made that forgiveness is a mini-miracle in its own right!
As for the Matthew passage: again, it is not about miracles. Jesus uses the "moving of the mountain" as hyperbole to underscore the more important lesson concerning prayer. It's as if His "argument" is as follows:
"So you think my cursing of the fig tree is a big deal? Hardly. The fig tree is small potatoes compared to a mountain, and undoubting faith can move mountains. Just practice believing prayer and whatever you ask you will receive. Now that's something to be amazed about!"
Neither passage you cite is about miracles. This is not to say God is no longer in the miracle-working business. Far from it.
You said something quite telling in your question, however:
". . . we can do miracles today?"
We cannot do miracles. Only God does miracles. Furthermore, His miracles are not flashy and pointless tricks such as uprooting trees and moving mountains. His true miracles, such as healing, for example, have a purpose, and that is to draw people to Himself in loving relationship.
"But [Jesus] answered and said unto them, 'An evil and adulterous generation seeketh after a sign; and there shall no sign be given to it, but the sign of the prophet Jonas:
The men of Nineveh shall rise in judgment with this generation, and shall condemn it: because they repented at the preaching of Jonas; and, behold , a greater than Jonas is here'" (Matthew 12:39-40 KJV).
Is it wrong, then, to want to see God perform miracles in the 21st century? No, not necessarily, as long as we remember that the greatest miracle of all occurs when a sinner repents and is forgiven, fully and freely, by Jesus! Now that's something amazing!