Bump main to 6.0.0~pre1

[chapulina]

Branch ign-gui5 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 #207 (1301cde) into main (9f63213) will not change coverage.
The diff coverage is n/a.

❗ Current head 1301cde differs from pull request most recent head df63cc5. Consider uploading reports for the commit df63cc5 to get more accurate results

@@           Coverage Diff           @@
##             main     #207   +/-   ##
=======================================
  Coverage   58.16%   58.16%           
=======================================
  Files          24       24           
  Lines        3069     3069           
=======================================
  Hits         1785     1785           
  Misses       1284     1284           

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[chapulina]

No unexpected failures, this is good to go!