move projVersionMajor to cpp

[vcloarec]

To allow calling to method of this class through the qgis core lib from third stand alone application.

Not sure because maybe there are some reasons to have the definition in the header...

backport to 3.18 ? (could be appreciate)

[vcloarec]

@nyalldawson , are you ok for a backport to 3.18 ?

[nyalldawson]

@vcloarec sure

[github-actions]

The backport to release-3_18 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release-3_18 release-3_18
# Navigate to the new working tree
cd .worktrees/backport-release-3_18
# Create a new branch
git switch --create backport-42158-to-release-3_18
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 e0489b3113774b6bb66426b9bf05e6d023c8d7b9
# Push it to GitHub
git push --set-upstream origin backport-42158-to-release-3_18
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-3_18

Then, create a pull request where the base branch is release-3_18 and the compare/head branch is backport-42158-to-release-3_18.