feat(ct): add SOLIDITY_UPGRADES document (#12272)

Adds SOLIDITY_UPGRADES.md to contracts-bedrock. Please note that
this document makes some references to a unified Solidity version.
This will be true in a few days...

packages/contracts-bedrock/README.md

@@ -16,6 +16,7 @@ High-level information about these contracts can be found within this README and
 - [Contributing](#contributing)
   - [Contributing Guide](#contributing-guide)
   - [Style Guide](#style-guide)
+  - [Solidity Versioning](#solidity-versioning)
 - [Deployment](#deployment)
   - [Deploying Production Networks](#deploying-production-networks)
 - [Generating L2 Genesis Allocs](#generating-l2-genesis-allocs)
@@ -71,6 +72,12 @@ Please refer to the [CONTRIBUTING.md](https://github.com/ethereum-optimism/optim
 OP Stack smart contracts should be written according to the [STYLE_GUIDE.md](./STYLE_GUIDE.md) found within this repository.
 Maintaining a consistent code style makes code easier to review and maintain, ultimately making the development process safer.
 
+### Solidity Versioning
+
+OP Stack smart contracts are designed to utilize a single, consistent Solidity version. Please
+refer to [SOLIDITY_UPGRADES.md](./SOLIDITY_UPGRADES.md) to understand the process for updating to
+newer Solidity versions.
+
 ## Deployment
 
 The smart contracts are deployed using `foundry`. The `DEPLOYMENT_OUTFILE` env var will determine the filepath that the

packages/contracts-bedrock/SOLIDITY_UPGRADES.md

+# Solidity Versioning Policy
+
+This document outlines the process for proposing and implementing Solidity version updates in the
+OP Stack codebase.
+
+## Unified Solidity Version
+
+The OP Stack codebase maintains a single, unified Solidity version across all contracts and
+components. This ensures consistency, simplifies maintenance, and reduces the risk of
+version-related issues.
+
+**Important**: New Solidity versions must not be introduced to any part of the codebase without
+going through the formal version update proposal process outlined in this document.
+
+## Update Process
+
+1. **Minimum Delay Period**: A new Solidity version must be at least 6 months old before it can be
+  considered for adoption.
+2. **Proposal Submission**: Before any Solidity version upgrades are made, a detailed proposal must
+  be submitted as a pull request to the [`ethereum-optimism/design-docs`][1] repository in the
+  `solidity/` subfolder, following the standardized format outlined below. This applies to the
+  entire codebase; individual components or contracts cannot be upgraded separately.
+3. **Review and Approval**: A dedicated review panel will assess the proposal based on the
+  following criteria:
+    - Is the Solidity version at least 6 months old?
+    - Does the proposed upgrade provide clear value to the codebase?
+    - Do any new features or bug fixes pose an unnecessary risk to the codebase?
+4. **Implementation**: If the proposal receives unanimous approval from the review panel, the
+  Solidity version upgrade will be implemented across the entire OP Stack codebase.
+
+## Proposal Submission Guidelines
+
+To submit a Solidity version upgrade proposal, create a new pull request to the
+[`ethereum-optimism/design-docs`][1] repository, adding a new file in the [`solidity/`][2]
+subfolder. Please use the dedicated [Solidity update proposal format][3]. Ensure that all sections
+are filled out comprehensively. Incomplete proposals may be delayed or rejected.
+
+## Review Process
+
+The review panel will evaluate each proposal based on the criteria mentioned in the "Review and Approval" section above. They may request additional information or clarifications if needed.
+
+## Implementation
+
+If approved, the Solidity version upgrade will be implemented across the entire OP Stack codebase. This process will be managed by the development team to ensure consistency and minimize potential issues. The upgrade will apply to all contracts and components simultaneously.
+
+<!-- References -->
+[1]: https://github.com/ethereum-optimism/design-docs
+[2]: https://github.com/ethereum-optimism/design-docs/tree/main/solidity
+[3]: https://github.com/ethereum-optimism/design-docs/tree/main/assets/solc-update-template.md