Bump main to 6.0.0~pre1

[chapulina]

Branch ign-sensors5 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 #119 (c5b628e) into main (8605a3a) will not change coverage.
The diff coverage is n/a.

❗ Current head c5b628e differs from pull request most recent head d6625b4. Consider uploading reports for the commit d6625b4 to get more accurate results

@@           Coverage Diff           @@
##             main     #119   +/-   ##
=======================================
  Coverage   76.13%   76.13%           
=======================================
  Files          23       23           
  Lines        2388     2388           
=======================================
  Hits         1818     1818           
  Misses        570      570           

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[chapulina]

No unexpected failures, this is good to go!