Added bootnodes for Rinkeby and Goerli

[nrednav]

This was requested here: #5629

I also:-

• Added the Ethereum Foundation's C++ bootnode for the Mainnet
• Added bootnode locations as comments, for Mainnet and Ropsten, as seen in Geth's bootnode list

[halfalicious]

@nrednav Thank you for your contribution 😄 Can you please also add a changelog entry to CHANGELOG.md: https://github.com/ethereum/aleth/blob/master/CHANGELOG.md

(Please add an "added" entry under the rest of the "added" entries here)

[nrednav]

@nrednav Thank you for your contribution 😄 Can you please also add a changelog entry to CHANGELOG.md: https://github.com/ethereum/aleth/blob/master/CHANGELOG.md

(Please add an "added" entry under the rest of the "added" entries here)

Hi, I'm quite new to all of this, would I submit another pull request after adding that entry to the changelog?

[chfast]

Make a new commit with the Changelog change and push it to the same branch.

[codecov-io]

Codecov Report

Merging #5634 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #5634      +/-   ##
==========================================
- Coverage   62.77%   62.74%   -0.03%     
==========================================
  Files         348      348              
  Lines       29648    29657       +9     
  Branches     3341     3341              
==========================================
- Hits        18612    18609       -3     
- Misses       9821     9830       +9     
- Partials     1215     1218       +3

[nrednav]

Make a new commit with the Changelog change and push it to the same branch.

Done, thank you : )

[gumb0]

I removed "Ethereum Foundation C++ Bootnode" as I don't think it's alive anymore.
(I tried it and it doesn't answer to Ping)