Update version to v2.2.1

Update version to v2.2.0

New features:
- DynamicBuffer can now be moved to and from String
- DynamicBuffer can be copied from SharedBuffer
- AsyncWebSocket interface is updated to use DynamicBuffer, clarifying
  ownership semantics

Fixes:
- contentTypeFor: Fix handling when extension not found
- Fix warnings on ESP32 when debug prints are enabled

Original repo links

Faster paralellism for LockedJsonResponse

LinkedLists: Remove extra indirection

This reduces the total number of allocations required for every request.
Also includes a pre-allocation optimization to
AsyncWebServerResponse::_assembleHead and some more migration to
PROGMEM strings.

Filename always with leading slash (Aircoookie#7)

Patch for use-after-free (upstream PR me-no-dev#952) (Aircoookie#6)

No file name for inline content disposition

2.0.4

Fix array pointer
Reduce string memory use

2.0.3