minetest -> core

[crabbymaniac]

• This PR replaces all the occurrences of depreciated minetest namespace and instead replaces them with core. Closes Replace minetest with core in the codebase #1395
• Command used to replace minetest -> core - find * -type f -exec grep -L 'http[s]*://[a-zA-Z0-9._-]*minetest\.' {} + | xargs sed -i 's/minetest\./core./g'

[farooqkz]

Checked apis and in ctf checked ctf_beds to ctf_rankings and LGTM. Didn't test the code.