OCT-1576 & OCT-1492: Fix altering migrations (#236)

## Description

## Definition of Done

1. [ ] Acceptance criteria are met.
2. [ ] PR is manually tested before the merge by developer(s).
    - [ ] Happy path is manually checked.
3. [ ] PR is manually tested by QA when their assistance is required
(1).
- [ ] Octant Areas & Test Cases are checked for impact and updated if
required (2).
4. [ ] Unit tests are added unless there is a reason to omit them.
5. [ ] Automated tests are added when required.
6. [ ] The code is merged.
7. [ ] Tech documentation is added / updated, reviewed and approved
(including mandatory approval by a code owner, should such exist for
changed files).
    - [ ] BE: Swagger documentation is updated.
8. [ ] When required by QA:
    - [ ] Deployed to the relevant environment.
    - [ ] Passed system tests.

---

(1) Developer(s) in coordination with QA decide whether it's required.
For small tickets introducing small changes QA assistance is most
probably not required.

(2) [Octant Areas & Test
Cases](https://docs.google.com/spreadsheets/d/1cRe6dxuKJV3a4ZskAwWEPvrFkQm6rEfyUCYwLTYw_Cc).

backend/migrations/versions/1f5a0f69733a_unify_proposals_to_projects.py

@@ -16,15 +16,9 @@
 
 def upgrade():
     with op.batch_alter_table("allocations", schema=None) as batch_op:
-        batch_op.add_column(
-            sa.Column("project_address", sa.String(length=42), nullable=False)
-        )
-        batch_op.drop_column("proposal_address")
+        batch_op.alter_column("proposal_address", new_column_name="project_address")
 
 
 def downgrade():
     with op.batch_alter_table("allocations", schema=None) as batch_op:
-        batch_op.add_column(
-            sa.Column("proposal_address", sa.VARCHAR(length=42), nullable=False)
-        )
-        batch_op.drop_column("project_address")
+        batch_op.alter_column("project_address", new_column_name="proposal_address")

backend/migrations/versions/58536423cfdd_unify_all_individual_rewards_to_vanilla_.py

@@ -16,15 +16,15 @@
 
 def upgrade():
     with op.batch_alter_table("pending_epoch_snapshots", schema=None) as batch_op:
-        batch_op.add_column(
-            sa.Column("vanilla_individual_rewards", sa.String(), nullable=False)
+        batch_op.alter_column(
+            column_name="all_individual_rewards",
+            new_column_name="vanilla_individual_rewards",
         )
-        batch_op.drop_column("all_individual_rewards")
 
 
 def downgrade():
     with op.batch_alter_table("pending_epoch_snapshots", schema=None) as batch_op:
-        batch_op.add_column(
-            sa.Column("all_individual_rewards", sa.VARCHAR(), nullable=False)
+        batch_op.alter_column(
+            column_name="vanilla_individual_rewards",
+            new_column_name="all_individual_rewards",
         )
-        batch_op.drop_column("vanilla_individual_rewards")