Point stale default branch references to "main"
Summary
The old references need updating to point to "main":
- First, the URL to the documentation itself reads "master": https://docs.rtems.org/branches/master/user/index.html
- "The Master branch the development branch and latest release is 5.3." on https://docs.rtems.org/
- "The Master branch the development branch and latest release is 5.3." on https://docs.rtems.org/branches.html
- "is one more than 6` and is on the ``master branch" on https://docs.rtems.org/branches/master/user/installation/developer.html (which should have been deleted)
- "the bug is not reproducible on the Git master" on https://docs.rtems.org/branches/master/user/installation/developer.html
- "Make sure not to commit changes in the master branch" on https://docs.rtems.org/branches/master/user/start/gsoc.html
- "The master branch has the version N.0.0 with N being the next major release number." on https://docs.rtems.org/branches/master/user/support/bugs.html
- "NOTE: The following example uses FreeBSD master branch" on https://docs.rtems.org/branches/master/user/exe/device-tree.html - FreeBSD's main branch seems to be called main including on freebsd.org