Mark old icu4c as conflicting (#17562)

GCC 4.8.5 on rhel6:
```
utext.cpp:572:5: error: 'max_align_t' in namespace 'std' does not name a
type
     std::max_align_t    extension;
     ^
utext.cpp: In function 'UText* utext_setup_67(UText*, int32_t,
UErrorCode*)':
utext.cpp:587:73: error: 'max_align_t' is not a member of 'std'
             spaceRequired = sizeof(ExtendedUText) + extraSpace -
sizeof(std::max_align_t);
                                                                         ^
utext.cpp:587:73: note: suggested alternative:
In file included from
/projects/spack/opt/spack/gcc-4.4.7/gcc/6ln2t7b/include/c++/4.8.5/cstddef:42:0,
                 from utext.cpp:19:
/projects/spack/opt/spack/gcc-4.4.7/gcc/6ln2t7b/lib/gcc/x86_64-unknown-linux-gnu/4.8.5/include/stddef.h:
425:3: note:   'max_align_t'
 } max_align_t;
   ^
utext.cpp:598:57: error: 'struct ExtendedUText' has no member named
'extension'
                 ut->pExtra    = &((ExtendedUText *)ut)->extension;
                                                         ^
   g++   ...  loadednormalizer2impl.cpp
   g++   ...  chariter.cpp
```

0 files changed, 0 insertions, 0 deletions