feat(manager/pep621)!: remove group name from depName (#28193)

Previously, the “depName” for pep621 was constructed using groupName/packageName, which in turn meant that the same dependency was upgraded in different branches if it was present in multiple groups. Instead, depName is now set to packageName. This will lead to a change of branch name for pep621 updates.

Closes #28131

BREAKING CHANGE: depName for pep621 dependencies changes, which will lead to branch name changes, which will lead to some autoclosing and reopening of PRs.