[TRIVIAL] Set version to 2.6.0-rc1

[legleux]

High Level Overview of Change

This PR only changes the version number. It will be merged as
soon as Github CI actions successfully complete.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.8%. Comparing base (c9d73b6) to head (da82e52).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #5672   +/-   ##
=======================================
  Coverage     78.8%   78.8%           
=======================================
  Files          814     814           
  Lines        71304   71306    +2     
  Branches      8350    8350           
=======================================
+ Hits         56184   56199   +15     
+ Misses       15120   15107   -13     

Files with missing lines	Coverage Δ
src/libxrpl/protocol/BuildInfo.cpp	98.2% <ø> (ø)

... and 5 files with indirect coverage changes

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