### What changes were proposed in this pull request?
`README#Build` and `sbt#System Requirements` extends to Scala 2.13.
### Why are the changes needed?
`README#Build` and `sbt#System Requirements`should extend to Scala 2.13 to align the SBT CI test results.
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
SBT CI tests.
Closes#1987 from SteNicholas/CELEBORN-987.
Authored-by: SteNicholas <programgeek@163.com>
Signed-off-by: Fu Chen <cfmcgrady@gmail.com>