Apply golangci-lint fixes

Add v2 migration guideIntroduce MIGRATION.md covering key v1 to v2 changes, link it from theREADME, and reference it in the 2.0.0 changelog.

Set release date

Set release date for beta.9

Fix unmarshaler offset error in struct fieldsWhen custom unmarshalers decoded container types in struct fields, thereflection decoder would fail with "no next offset available" when tryingto advance to the next field.

Make capitalization of uint consistent

Prepare for 2.0.0-beta.5

Add fuzzing tests for MMDB parsing

2.0.0-beta.4- **BREAKING CHANGE**: Removed experimental `deserializer` interface and  supporting code. Applications using this interface should migrate to the  `Unmarshaler` interface by implementing `UnmarshalMaxMindDB(d *Decoder) error`  instead.- `Open` and `FromBytes` now accept options.- **BREAKING CHANGE**: `IncludeNetworksWithoutData` and `IncludeAliasedNetworks`  now return a `NetworksOption` rather than being one themselves. These must now  be called as functions: `Networks(IncludeAliasedNetworks())` instead of  `Networks(IncludeAliasedNetworks)`. This was done to improve the documentation  organization.- Added `Unmarshaler` interface to allow custom decoding implementations for  performance-critical applications. Types implementing  `UnmarshalMaxMindDB(d *Decoder) error` will automatically use custom decoding  logic instead of reflection, following the same pattern as  `json.Unmarshaler`.- Added public `Decoder` type and `Kind` constants in `mmdbdata` package for  manual decoding. `Decoder` provides methods like `ReadMap()`, `ReadSlice()`,  `ReadString()`, `ReadUInt32()`, `PeekKind()`, etc. `Kind` type includes  helper methods `String()`, `IsContainer()`, and `IsScalar()` for type  introspection. The main `maxminddb` package re-exports these types for  backward compatibility. `NewDecoder()` supports an options pattern for  future extensibility.- Enhanced `UnmarshalMaxMindDB` to work with nested struct fields, slice  elements, and map values. The custom unmarshaler is now called recursively  for any type that implements the `Unmarshaler` interface, similar to  `encoding/json`.- Improved error messages to include byte offset information and, for the  reflection-based API, path information for nested structures using JSON  Pointer format. For example, errors may now show "at offset 1234, path  /city/names/en" or "at offset 1234, path /list/0/name" instead of just the  underlying error message.- **PERFORMANCE**: Added string interning optimization that reduces allocations  while maintaining thread safety. Reduces allocation count from 33 to 10 per  operation in downstream libraries. Uses a fixed 512-entry cache with per-entry  mutexes for bounded memory usage (~8KB) while minimizing lock contention.

Update license info in README.md