Add 2025-26 Scottish income tax rate thresholds (#1473)

Update Scottish income tax thresholds effective from 2025-04-06:
- Starter/Basic rate threshold: £2,827
- Intermediate rate threshold: £14,921
- Higher rate threshold: £31,092 (frozen)
- Advanced rate threshold: £62,430 (frozen)
- Top rate threshold: £125,140 (frozen)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: PavelMakarchuk

changelog_entry.yaml

@@ -0,0 +1,4 @@
+- bump: minor
+  changes:
+    added:
+    - 2025-26 Scottish income tax rates.

policyengine_uk/parameters/gov/hmrc/income_tax/rates/scotland/rates.yaml

@@ -18,6 +18,7 @@ brackets:
         2022-04-06: 2_162
         2023-04-06: 2_162
         2024-04-06: 2_306
+        2025-04-06: 2_827
       metadata:
         uprating: gov.economic_assumptions.indices.obr.consumer_price_index
     rate:
@@ -30,6 +31,7 @@ brackets:
         2022-04-06: 13_118
         2023-04-06: 13_118
         2024-04-06: 13_991
+        2025-04-06: 14_921
       metadata:
         uprating: gov.economic_assumptions.indices.obr.consumer_price_index
     rate:
@@ -42,6 +44,7 @@ brackets:
         2022-04-06: 31_092
         2023-04-06: 31_092
         2024-04-06: 31_092
+        2025-04-06: 31_092
       metadata:
         uprating: gov.economic_assumptions.indices.obr.consumer_price_index
     rate:
@@ -53,6 +56,7 @@ brackets:
         2017-04-06: 150_000
         2023-04-06: 125_140
         2024-04-06: 62_430
+        2025-04-06: 62_430
       metadata:
         uprating: gov.economic_assumptions.indices.obr.consumer_price_index
     rate:
@@ -64,6 +68,7 @@ brackets:
       values:
         2017-04-06: null
         2024-04-06: 125_140
+        2025-04-06: 125_140
       metadata:
         uprating: gov.economic_assumptions.indices.obr.consumer_price_index
     rate: