Bump protobuf from 3.13.0 to 3.15.6

[dependabot]

Bumps protobuf from 3.13.0 to 3.15.6.

Release notes

Sourced from protobuf's releases.

Protocol Buffers v3.15.6

Ruby

• Fixed bug in string comparison logic (#8386)

Protocol Buffers v3.15.5

Ruby

• Fixed quadratic memory use in array append (#8379)

PHP

• Fixed quadratic memory use in array append (#8379)

C++

• Do not disable RTTI by default in the CMake build (#8377)

Protocol Buffers v3.15.4

Ruby

• Fixed SEGV when users pass nil messages (#8363)
• Fixed quadratic memory usage when appending to arrays (#8364)

C++

• Create a CMake option to control whether or not RTTI is enabled (#8361)

PHP

• read_property() handler is not supposed to return NULL (#8362)

Protocol Buffers v3.15.3

Ruby

• Ruby <2.7 now uses WeakMap too, which prevents memory leaks. (#8341)

Protocol Buffers v3.15.2

Ruby

• Fix for FieldDescriptor.get(msg) (#8330)

C++

• Fix PROTOBUF_CONSTINIT macro redefinition (#8323)

Protocol Buffers v3.15.1

Ruby

• Bugfix for Message.[] for repeated or map fields (#8313)
• Fix for truncating behavior when converting Float to Duration (#8320)

C++

• Small fixes for MinGW and for C++20 with GCC (#8318)

Protocol Buffers v3.15.0

Protocol Compiler

• Optional fields for proto3 are enabled by default, and no longer require the --experimental_allow_proto3_optional flag.

C++

... (truncated)

Commits

• 6aa539b Update protobuf version
• 971a593 Updated CHANGES.txt and PHP changelog for 3.15.6
• b2991b8 Merge pull request #8386 from haberman/ruby-string-compare
• bb322c2 [Ruby] Fixed bug in string comparison logic.
• 4fff47a Update protobuf version
• 5702d51 Updated CHANGES.txt and PHP release notes for 3.15.5
• b9a036b Do not disable RTTI by default in the CMake build
• 72b02b5 Merge pull request #8379 from haberman/quadratic-mem-fix
• 9b5e357 Fixed quadratic memory use in array append for PHP and Ruby.
• fd8aabf Update protobuf version
• Additional commits viewable in compare view

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[dependabot]

Looks like protobuf is up-to-date now, so this is no longer needed.