[dns] fix otDnsEncodeTxtData parameter type mismatch (#13288)

This commit fixes a parameter type mismatch between declaration and
implementation of function `otDnsEncodeTxtData()`.

`include/openthread/dns.h` provides an API `otDnsEncodeTxtData()` for
encoding TXT record entries list into TXT data, which is implemented
in `src/core/api/dns_api.cpp`. The problem is, `otDnsEncodeTxtData()`
is declared with `uint16_t aNumTxtEntries` in the public header, but
defined with `uint8_t aNumTxtEntries` in the implementation. This
mismatch can lead to symbol/ABI inconsistency for C callers and causes
link failure in downstream integrations.

This commit changes the type of parameter `aNumTxtEntries` in cpp
implementation from `uint8_t` to `uint16_t`, to follow up type
definitions in the declaration and later called
`Dns::TxtEntry::AppendEntries()`. So that downstream integrations will
not run into link failure when using `otDnsEncodeTxtData()`.

src/core/api/dns_api.cpp

@@ -49,7 +49,7 @@ otError otDnsGetNextTxtEntry(otDnsTxtEntryIterator *aIterator, otDnsTxtEntry *aE
 }
 
 otError otDnsEncodeTxtData(const otDnsTxtEntry *aTxtEntries,
-                           uint8_t              aNumTxtEntries,
+                           uint16_t             aNumTxtEntries,
                            uint8_t             *aTxtData,
                            uint16_t            *aTxtDataLength)
 {