SRTMHGT .hgt.gz cannot be opened

[Even Rouault]

The SRTMHGT driver doesn't recognize the .hgt.gz extension when opening something like /vsigzip/N37W105.hgt.gz

[Even Rouault]

In 39917:

SRTMHGT: recognizes the .hgt.gz extension (fixes #7016)

[Even Rouault]

In 39918:

/vsigzip/: avoid trying to write a .gz.properties file on a /vsicurl/ file (refs #7016)

[Even Rouault]

In 39919:

SRTMHGT: recognizes the .hgt.gz extension (fixes #7016)

[Even Rouault]

In 39920:

/vsigzip/: avoid trying to write a .gz.properties file on a /vsicurl/ file (refs #7016)