Bump main to 6.0.0~pre1

[chapulina]

Branch ign-gazebo5 has been created for the Edifice release and 5.0.0 released from that.

The main branch now corresponds to version 6.0.0~pre1, and nightlies will soon be created from this branch.

gazebo-tooling/release-tools#421

[codecov]

Codecov Report

Merging #735 (2b3ff5e) into main (7e157a4) will increase coverage by 0.03%.
The diff coverage is n/a.

❗ Current head 2b3ff5e differs from pull request most recent head 649dc67. Consider uploading reports for the commit 649dc67 to get more accurate results

@@            Coverage Diff             @@
##             main     #735      +/-   ##
==========================================
+ Coverage   65.31%   65.34%   +0.03%     
==========================================
  Files         240      240              
  Lines       17602    17602              
==========================================
+ Hits        11496    11502       +6     
+ Misses       6106     6100       -6     

Impacted Files	Coverage Δ
src/SimulationRunner.cc	93.77% <0.00%> (+1.03%)	⬆️

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[chapulina]

No unexpected failures, this is good to go!