Update Lisk SDK to v6.0.0-rc.4

[sameersubudhi]

What was the problem?

This PR resolves #874

How was it solved?

• Update network version to 5.0 for all networks
• Upgrade Lisk SDK to v6.0.0-rc.4
• Bump Lisk Core version to v4.0.0-rc.7
• Update docs
• Fix broken links in README

How was it tested?

Locally with:

• yarn install --frozen-lockfile
• make build
  • Installs local dependencies and build docker images
• rm -rf ~/.lisk
• ./bin/run start -n devnet

[codecov]

Codecov Report

Merging #875 (8df4a9d) into release/4.0.0 (0ccff65) will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff               @@
##           release/4.0.0     #875   +/-   ##
==============================================
  Coverage          96.89%   96.89%           
==============================================
  Files                 14       14           
  Lines                258      258           
  Branches              14       14           
==============================================
  Hits                 250      250           
  Misses                 8        8           

[nagdahimanshu]

Looks good and works as expected 👍