In README, move outdated specs to new section

[jtraglia]

I think it would be easier to read if these three specs were in a different table.

I also took this as an opportunity to clean up these tables some. Notably:

• Change "EIP-6110 fork" to "Electra fork".
• Add networking changes to Electra items.
• Use same table structure for stable & in-development spec tables.
  • This will allow an easy copy/paste when this becomes stable.
• Fix a small mistake, an extra <ul> (unordered list) section starting with Capella.
  • See the extra space after "light client sync protocol changes" in Capella vs Altair.

[ralexstokes]

looks fine from a quick skim