Bump Arrow to 7.0.0

[mbrobbel]

bors r+

[bors[bot]]

Timed out.

[mbrobbel]

bors retry

[bors[bot]]

Build succeeded:

• Book
• C++ (codegen/cpp/fletchgen)
• CentOS (7, runtime/cpp)
• C++ (fletchgen, codegen/cpp/fletchgen)
• Check Markdown links
• C++ (platforms/echo/runtime)
• C++ (runtime/cpp)
• C++ (runtime, runtime/cpp)
• Linux (runtime/cpp)
• Prepare
• Python (codegen/python)
• Python (codegen/python, 2014, cp37-cp37m)
• Python (codegen/python, 2014, cp38-cp38)
• Python (codegen/python, 2014, cp39-cp39)
• Python (runtime/python)
• Python (runtime/python, 2014, cp37-cp37m)
• Python (runtime/python, 2014, cp38-cp38)
• Python (runtime/python, 2014, cp39-cp39)
• Test (codegen/test/listprim)
• Test (codegen/test/primmap)
• Test (codegen/test/stringread)
• Test (examples/stringwrite/hardware)
• Test (examples/sum/hardware)
• Ubuntu (18.04, codegen/cpp/fletchgen)
• Ubuntu (18.04, runtime/cpp)
• Ubuntu (20.04, codegen/cpp/fletchgen)
• Ubuntu (20.04, runtime/cpp)
• VHDL