Hold off on tagging 28.0

[JeremyRand]

@domob1812 please hold off on tagging nc28.0 until I can make sure a few potential blockers are dealt with.

[JeremyRand]

Potential blockers:

• Default IP and onion P2P ports conflict with each other, and with Bitcoin Core default onion P2P port #545
• Making sure Guix builds succeed and are reproducible.
• Asking our QA team to briefly try the Guix binaries.

[JeremyRand]

Other potential blockers:

• Making sure Guix builds on ppc64le are reproducible with x86_64.
• Update the seed node list.

[JeremyRand]

@domob1812 can you cherry-pick #553 and #557 to the 28.x branch?

Other remaining blockers for tagging 28.0:

• Making sure Guix builds on ppc64le are reproducible with x86_64.
  • This looks like there was a regression upstream between 27.x and 28.x. I've reported a bug upstream today. I will re-evaluate whether I want this to block a release once upstream responds, and once I talk with our other Guix builders.
• Making sure Guix builds succeed and are reproducible.
• Asking our QA team to briefly try the Guix binaries.
• Update the seed node list (including mainnet DNS seeds).

[domob1812]

Done the backports of #553 and #557.