If Bitcoin removes the 21 million supply cap, will it still be called bitcoin?
No. It'd be an altcoin.
I mean if the miners agree with the proposal and increase the supply limit to a higher number, technically it will still be bitcoin because the miners have the power to change the network rules.
The miners cannot do whatever they want with the consensus rules. The fact that the network relies on them does not mean they have any power beyond choosing the order of the transactions. Just remember what happened in 2017 when they tried to increase the block size from 1 MB to 32 MB. It
didn't go well.