6866259 net: Hardcoded seeds update for 0.21 (Wladimir J. van der Laan)
36e875b contrib: Add new versions to makeseeds.py and update gitignore (RandyMcMillan)

Pull request description:

  Stats:

  ```
    IPv4   IPv6  Onion Pass
  426728  59523   7900 Initial
  426728  59523   7900 Skip entries with invalid address
  426728  59523   7900 After removing duplicates
  426727  59523   7900 Skip entries from suspicious hosts
  123226  51785   7787 Enforce minimal number of blocks
  121710  51322   7586 Require service bit 1
    4706   1427   3749 Require minimum uptime
    4124   1098   3681 Require a known and recent user agent
    4033   1075   3681 Filter out hosts with multiple bitcoin ports
     512    140    512 Look up ASNs and limit results per ASN and per net
  ```
  I've credited RandyMcMillan for the first commit because of #20190.

  There are at least enough onions this time! Number of IPv6 nodes that pass all the requirements seems similar to last time in #18506.

  For the next major release we'll want TORv3 hardcoded peers as well. This makes no sense now as there are hardly any. But it'd make sense to think about how to collect them because they cannot come from the DNS seeds.

  ### Reviewing
  ```
  2020-10-28 12:04:45     jnewbery  wumpus: Do you have any suggestions for how to review #20237 ?
  2020-10-28 12:28:37     wumpus  jnewbery: previous PRs like it might be a guide there (#18506, #16999), e.g. people could try to repeat the last step in https://github.com/bitcoin/bitcoin/tree/master/contrib/seeds#seeds and see if it ends up with the same .h file, you could also repeat the entire process but as the list of peers from the seeder will be different every time that will give a (slightly, hopefully)
  2020-10-28 12:28:37     wumpus  different output
  2020-10-28 12:49:40     wumpus  testing what part of the peers are connectable is also useful
  2020-10-28 12:51:05     wumpus  or to go deeper, whether most part of the nodes are 'good nodes' and not say spy nodes, but i don't know what means of testing
  ```

ACKs for top commit:
  jonatack:
    ACK 6866259

Tree-SHA512: 6b913ec92932de03304301a0cbf7b4a912ed09d890b019deeb449b8fa787c4994222368c6bf08b3c6e2bfa474442612e1c9de9327ec46ba59c37a5f38af50c75