Today is not necessarily the present. Today signifies the present in the first sentence, since the sentence would be invalid/ungrammatical with an adverb of time that is completely in the past (example: yesterday).
However, since today lasts 24 hours, the use of 'today' in the second sentence (using the simple past) refers to earlier today or earlier in the day, which is in the past. Thus the same word today is conceived of in two different ways.
The use of the present perfect is often used to report the latest news. Its effect is to make the news seem more urgent to listeners or readers.
The use of the simple past reports the event as something that is over and done with, that is, an event that took place in the past.
Again, either verb tense is fine, because today is being used differently.
Another example is
I had a really good day today.
Although "today" is not over, the speaker is referring to the part of the 24-hour period of today that is in the past.
I have had a really good day today.
Using the present perfect in this sentence, the adverb of time today must include the moment of speaking (that is, the present).