deps: update dependency com.google.protobuf:protobuf-bom to v3.12.2

[renovate-bot]

This PR contains the following updates:

Package	Update	Change
com.google.protobuf:protobuf-bom (source)	patch	3.12.0 -> 3.12.2

---

Release Notes

protocolbuffers/protobuf

v3.12.2

Compare Source

C++

• Simplified the template export macros to fix the build for mingw32. (#​7539)

Objective-C

• Fix for the :protobuf_objc target in the Bazel BUILD file. (#​7538)

v3.12.1

Compare Source

Ruby

• Re-add binary gems for Ruby 2.3 and 2.4. These are EOL upstream, however
  many people still use them and dropping support will require more
  coordination. (#​7529, #​7531).

---

Renovate configuration

📅 Schedule: At any time (no schedule defined).

🚦 Automerge: Disabled by config. Please merge this manually once you are satisfied.

♻️ Rebasing: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 Ignore: Close this PR and you won't be reminded about this update again.

---

• If you want to rebase/retry this PR, check this box

---

This PR has been generated by WhiteSource Renovate. View repository job log here.

[gcf-merge-on-green]

Merge-on-green attempted to merge your PR for 6 hours, but it was not mergeable because either one of your required status checks failed, or one of your required reviews was not approved. Learn more about your required status checks here: https://help.github.com/en/github/administering-a-repository/enabling-required-status-checks. You can remove and reapply the label to re-run the bot.

[gcf-merge-on-green]

Merge-on-green attempted to merge your PR for 6 hours, but it was not mergeable because either one of your required status checks failed, or one of your required reviews was not approved. Learn more about your required status checks here: https://help.github.com/en/github/administering-a-repository/enabling-required-status-checks. You can remove and reapply the label to re-run the bot.