proc-macro2 is hard pinned in arrow-flight

[alamb]

Describe the bug

Pinning the version of proc-macro used in arrow-flight prevents updates of the proc-macro crate.

In the meantime, an indirect dependency (quote) released a version that was not strictly backwards compatible and didn't work with this version of proc-macro2. The issue was subsequently fixed in newer versions of the quote but the old version is still on crates.io, preventing arrow from compiling. dtolnay/quote#204 requests to author to yank it.

Trying to publish arrow-flight results in the following error:

   Compiling prost-build v0.8.0
error[E0599]: no function or associated item named `from_str_unchecked` found for struct `proc_macro2::Literal` in the current scope
   --> /Users/alamb/.cargo/registry/src/github.com-1ecc6299db9ec823/quote-1.0.11/src/runtime.rs:301:41
    |
301 |         let literal = unsafe { Literal::from_str_unchecked(repr) };
    |                                         ^^^^^^^^^^^^^^^^^^ function or associated item not found in `proc_macro2::Literal`

error[E0599]: no function or associated item named `from_str_unchecked` found for struct `proc_macro2::Literal` in the current scope
   --> /Users/alamb/.cargo/registry/src/github.com-1ecc6299db9ec823/quote-1.0.11/src/runtime.rs:310:45
    |
310 |         let mut literal = unsafe { Literal::from_str_unchecked(repr) };
    |                                             ^^^^^^^^^^^^^^^^^^ function or associated item not found in `proc_macro2::Literal`

For more information about this error, try `rustc --explain E0599`.
error: could not compile `quote` due to 2 previous errors
warning: build failed, waiting for other jobs to finish...
error: failed to verify package tarball

Caused by:
  build failed

It also causes errors in downstream dependencies such as Datafusion, such as apache/datafusion#1498 (when a bugfix to a dependent crate requires upgrading to a newer version of proc-macro2

To Reproduce
See apache/datafusion#1498

Expected behavior
Be as flexible with the versions allowed as possible

Additional context
Add any other context about the problem here.

[alamb]

Filed request to yank broken crates.io release: dtolnay/quote#204