deps: use google-cloud-spanner-bom instead of libraries-bom

[suztomo]

@olavloite, @stephaniewang526, @elharo, @chingor13

As this repository uses the shared dependencies BOM now, google-cloud-spanner-bom has the dependencies necessary to replace the libraries-bom. It's more smaller and will not cause the dependency conflicts that are irrelevant to this repository (motivation: #251 (comment)).

---

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #252 ☕️

[codecov]

Codecov Report

Merging #258 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #258   +/-   ##
=========================================
  Coverage     71.40%   71.40%           
  Complexity     1104     1104           
=========================================
  Files            24       24           
  Lines          3396     3396           
  Branches        525      525           
=========================================
  Hits           2425     2425           
  Misses          756      756           
  Partials        215      215           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6c2ff16...895a9e6. Read the comment docs.