build: Bump version to 0.20.99
build: Bump version to 0.20.99

Now that 0.20 branch has been split off, master is 0.20.99 (pre-0.21).

Also clean out release notes.

Tree-SHA512: bba6133ae9708f75206c8934901b9f9909a233330f4dfefb3c24175bf8e11631cdc89a5d24a22421a73083f7eb743e977db8020b87dfbd3c1e6043929a19a285

Former-commit-id: 62f5688a4e4ce4f833e432875f0eeaf00a584476
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin