Migrate out betavoltaic item

[chaosvolt]

Summary

SUMMARY: Balance "Fix spawn of now-useless betavoltiac item, migrate it out"

Purpose of change

Fixes for stuff overlooked in a PR I merged, hecc: #2725

Describe the solution

1. Added a migration converting betavolatics to plutonium cells.
2. Set centrifuges to yield plutonium cells instead of betavolatics on deconstruct.
3. Updated a flier snippet to not refer to any specific item now and instead just nuclear energy in general.

Describe alternatives you've considered

Screaming. More seriously, also migrating the RTG item at some point, assuming I don't find a way to use that to make plutonium generators a valid grid furniture balanced towards a low level of infinite power instead of burning plutonium cells (in which case I'd still use the minireactor for that).

Testing

1. Checked affected files for syntax and lint errors, and searched for any other hints of the item in JSON and mods folders.
2. Started up a save in an older release, spawning in 5 betavoltaics in my inventory and 5 on the ground.
3. Saved and copied save folder to compiled test build in this branch.
4. Loaded it up, obtained a total of 50 plutonium cells.

Additional context

[scarf005]

looks good. could you

1. mention PR number in purpose of change
2. test if betavoltaic cells are correctly migrated?

[chaosvolt]

And tested with 10 cells, each one turns into an entire stack of the default 5 cells each (with any in your inventory merging stacks, loose ones on the ground don't).

[scarf005]

Before migration	After migration

1 betavoltaic cells are converted to 5 plutonium cells. i guess that's okay conversion ratio considering it's only done once and is beta cells being quite scarce.

deconstructing centrifuge gives intended plutonium cells.