[giammin]

Adds an "Upgrading from v1 to v2" section to the README, built from the changes between v1.1.4 and main.

• Breaking changes, each with its scope and migration step:
  • IDbDataProtectionStorage requires Delete.
  • Schema/table name validation at startup (PostgreSQL names lowercase only).
  • The global Dapper underscore mapping is no longer set.
  • The PostgreSQL NpgsqlDataSource is a keyed service.
  • StorageWrapper<TStorage> is internal.
  • LoggingExtensions is internal, and the Value log property is removed.
• Behavior changes:
  • The SQL Server friendly-name index is created on existing tables (with a query to find duplicates).
  • UTC InsertDate.
  • A missing schema is created.
  • Blank friendly names become NULL.
  • Malformed keys are skipped.
  • KeyInsertException no longer contains the key XML.
  • Key deletion is enabled.
• Packaging: DataProtection 10.0.13, no direct System.Security.Cryptography.Xml reference, no symbols package.

Closes #44