In 2020, the developers announced a major update for Minecraft: Java Edition. Players will soon have to transfer their Mojang account into an Microsoft account to continue playing. Although it's still an incredibly long time to wait the developers have confirmed the date and issued the final call for account migration.
As as of March 10, you'll no longer be able play Minecraft: Java Edition with the Mojang account. The developers want all users to switch to Microsoft accounts for "improved security and player security" and are entice people to make the change with the promise of free Minecraft capes.
You can find all the details on how to migrate your account here. Java Edition will continue to be supported as usual after the change and will continue to remain a separate entity from Bedrock. This is a simple unification of the accounts systems. You can still use your previous username. minecraft servers need is an Xbox gamertag.