hotfix: revert to 1.14 in plugin.yml to be backward-compatible again.

- Fix infinite momentum bug due to minecraft's minecart movement code being a hot mess. See GitHub issue if you're curious about the technical details.
- Default hard brake block changed from obsidian to sand soul. This souldn't affect existing configurations.
- Speed multiplier range now starts from 1 instead of 0.
- Hard brake fallback value now 8 instead of 1 when specifying invalid values in config.
- Build target changed from Java 8 to Java 11

Updated Spigot API to 1.20.2. This enables the use of newer materials as boost/brake blocks. Backwards compatible down to Minecraft 1.14

Added hard brake block:

- hard braking is achieved using unpowered power rail placed on obisidian (default, configurable)

- harder deceleration by factor of 8 (configurable)

- Added support to set any one block as boost block.
- Added "any" as option for boost block to make every powered rail a high speed powered rail.

Added config option to change boost block. Previously you had to use redstone blocks for high speed rails, but now it is configurable.

Options:

• minecraft:redstone_block
• minecraft:lapis_block

- Added notification message for multipliers >4x

- Lowered multiplier max limit from 50x to 8x due to servers not being able to realistically keep up with multipliers higher than 8.
- Fixed bug where you could set arbitrary multiplier via config.