PG: fix default value for IDENTITY PKs

[elpaso]

Fixes #29560 - PostgreSQL identity column not recognized properly

[qgis-bot]

The backport to release-3_10 failed:

The process 'git' failed with exit code 128

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_10 release-3_10
# Navigate to the new working tree
cd .worktrees/backport-release-3_10
# Create a new branch
git switch --create backport-34017-to-release-3_10
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick ec4d9f6871f52ffc2557866df71e5693b13ed047
# Push it to GitHub
git push --set-upstream origin backport-34017-to-release-3_10
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-3_10

Then, create a pull request where the base branch is release-3_10 and the compare/head branch is backport-34017-to-release-3_10.

[nyalldawson]

Looks like this change broke the Travis build -- can you investigate?

[elpaso]

I see Travis green here, where should I look for the error?

[elpaso]

@nyalldawson looking at it...