[16.0][FIX]purchase_manual_delivery: qty compute fixes

[ChristophAbenthungCibex]

Fixes the following 2 errors:

1. _compute_existing_qty not recomputed when changing product_uom_qty
2. Line is shown as pending_to_receive when existing_qty is higher than product_uom_qty

How-to replicate 2.:

1. Login as admin
2. Go to "Purchase"
3. Create a new one
4. Add any "Vendor" and set "Purchase manual delivery" to True
5. Add any product with "Quantity" = 5
6. Save and "Confirm order"
7. Click "Create incoming shipment"
8. Change "Quantity to 4"
9. Click "Create and view picking"
10. Change value of "Done" to 6 (any amount higher than the original expected (product_uom_qty) is fine)
11. Validate and go back to the Purchase Order
12. The Purchase Order Line is now highlighted blue because its value for pending_to_receive is True

[HviorForgeFlow]

/ocabot merge patch

[OCA-git-bot]

@HviorForgeFlow The merge process could not start, because command `git merge --no-ff -m 'Merge PR #2380 into 16.0

Signed-off-by HviorForgeFlow' tmp-pr-2380` failed with output:

Auto-merging purchase_manual_delivery/models/purchase_order.py
CONFLICT (content): Merge conflict in purchase_manual_delivery/models/purchase_order.py
Automatic merge failed; fix conflicts and then commit the result.

[HviorForgeFlow]

@ChristophAbenthungCibex can you rebase this branch? Conflict due to https://github.com/OCA/purchase-workflow/pull/2385

[ChristophAbenthungCibex]

@ChristophAbenthungCibex can you rebase this branch? Conflict due to #2385

@HviorForgeFlow Done.

[HviorForgeFlow]

/ocabot merge patch

[OCA-git-bot]

Hey, thanks for contributing! Proceeding to merge this for you. Prepared branch 16.0-ocabot-merge-pr-2380-by-HviorForgeFlow-bump-patch, awaiting test results.

[OCA-git-bot]

Congratulations, your PR was merged at 436e979a6aba541f0e1999af184bd8d3b04e3908. Thanks a lot for contributing to OCA. ❤️