Way back in 2020, the developers announced a major change to Minecraft: Java Edition, as players would soon be required to swap from the Mojang account to an Microsoft account to continue playing. Minecraft pixelmon servers is "soon" in relative terms, as we're not yet at the deadline however, the developers have just confirmed the date and put out the final call for account migration.
You'll no longer be able to play Minecraft: Java Edition if you have an account with Mojang account. For "improved security, player safety" the developers would like everyone to switch to Microsoft accounts. They are offering free Minecraft caps to encourage people to do so.
Here are the complete details on how to transfer your account. Java Edition will be supported as normal following the change and will remain a separate entity from Bedrock. This is a pure unification of the account systems. You can use your old username. All you need is an Xbox gamertag.