Bump protobuf from 3.5.2 to 3.15.4

[dependabot-preview]

Bumps protobuf from 3.5.2 to 3.15.4.

Release notes

Sourced from protobuf's releases.

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++

• MessageDifferencer: fixed bug when using custom ignore with multiple unknown fields
• Use init_seg in MSVC to push initialization to an earlier phase.
• Runtime no longer triggers -Wsign-compare warnings.
• Fixed -Wtautological-constant-out-of-range-compare warning.
• DynamicCastToGenerated works for nullptr input for even if RTTI is disabled
• Arena is refactored and optimized.
• Clarified/specified that the exact value of Arena::SpaceAllocated() is an implementation detail users must not rely on. It should not be used in unit tests.
• Change the signature of Any::PackFrom() to return false on error.
• Add fast reflection getter API for strings.
• Constant initialize the global message instances
• Avoid potential for missed wakeup in UnknownFieldSet

... (truncated)

Commits

• fd8aabf Update protobuf version
• e5c067f Updated CHANGES.txt and PHP changelog for 3.15.4 release
• aa41d90 read_property() handler is not supposed to return NULL
• cf7d81f Merge pull request #8363 from haberman/ruby-nil-fix
• 7d63b99 Removed compatibility test that tries to append "nil" to repeated field.
• 1add7a7 Create a CMake option to control whether or not RTTI is enabled
• dfa5457 [Ruby] Fixed SEGV when users pass nil messages.
• 983d115 Update protobuf version
• eb542e6 Updated CHANGES.txt for 3.15.3 release
• 80ec787 Merge pull request #8341 from haberman/ruby-2.5-gc
• Additional commits viewable in compare view

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[dependabot-preview]

Superseded by #157.