Bump tx version.

[Neopallium]

Because v6.0 is a major breaking release, we need to bump the chain transaction_version.

[codecov[bot]]

Codecov Report

Merging #1496 (a7614d7) into develop (9c6e6b2) will increase coverage by 0.01%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #1496      +/-   ##
===========================================
+ Coverage    54.12%   54.14%   +0.01%     
===========================================
  Files          145      145              
  Lines        32401    32401              
===========================================
+ Hits         17536    17542       +6     
+ Misses       14865    14859       -6     

Impacted Files	Coverage Δ
pallets/runtime/develop/src/runtime.rs	3.65% <ø> (ø)

... and 3 files with indirect coverage changes