meta: add SENTRY_DEPRECATED(msg) macro

[jpnurmi]

[build] sentry-native/examples/example.c: In function ‘main’:
[build] sentry-native/examples/example.c:384:5: warning: ‘sentry_sdk_name’ is deprecated: Use `sentry_options_get_sdk_name` instead [-Wdeprecated-declarations]
[build]   384 |     const char *foo = sentry_sdk_name();
[build]       |     ^~~~~
[build] In file included from sentry-native/examples/example.c:9:
[build] sentry-native/include/sentry.h:2593:37: note: declared here
[build]  2593 | SENTRY_EXPERIMENTAL_API const char *sentry_sdk_name(void);
[build]       |                                     ^~~~~~~~~~~~~~~

Docs:

• GCC 4.5: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html
• Clang 3.0: https://clang.llvm.org/docs/AttributeReference.html#deprecated
• MSVC 2005 (8.0): https://learn.microsoft.com/en-us/cpp/cpp/declspec

Close: #1307

[supervacuus]

None of these compilers could compile crashpad, but I still appreciate the version checks.