Limit the run of draftversion-check to the relevant branches

[vhscampos]

The draftversion checker must run only in the main branch. This patch limits the run conditions to abide by this.

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name: Pull request
about: Technical issues, document format problems, bugs in scripts or feature proposal.

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• If an issue reporting the bug exists, I have mentioned it in the
  PR (do not bother creating the issue if all you want to do is
  fixing the bug yourself).
• I have added/updated the SPDX-FileCopyrightText lines on top
  of any file I have edited. Format is SPDX-FileCopyrightText: Copyright {year} {entity or name} <{contact informations}>
  (Please update existing copyright lines if applicable. You can
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  commas to separate gaps, as in 2018-2020, 2022).
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  specification I have edited (this will show up in the text
  rendered in the PDF and other output format supported). The
  format is the same described in the previous item.
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  patch will be automatically checked in the Actions of the pull
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• I have added an item that describes the changes I have
  introduced in this PR in the section Changes for next
  release of the section Change Control/Document history
  of the document. Create Changes for next release if it does
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  do not need to be listed.
• When modifying content and/or its rendering, I have checked the
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• The variable draftversion is set to true in the YAML header
  of the sources of the specifications I have modified.
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