[OTA Server] change "location" parameter code once CHAR_STRING is supported

[holbrookt]

Problem

emberAfOtaSoftwareUpdateServerClusterQueryImageCallback() contains some bandaid code to get the length of the location parameter, since it is being generated as uint8_t* instead of Span:

    const uint8_t locationLen = emberAfStringLength(location);
    if ((locationLen != kLocationParamLength) || (metadataForServer.size() > kMaxMetadataLen))

(This was added in #6476)

This may need to happen in the same PR as when CHAR_STRING is supported, since it may not compile afterwards.

Proposed Solution

After CHAR_STRING is supported, location should be a Span object, and the size() method can be used instead.