PHPLIB-1128: Move generate_index_name() to private method within IndexInput

[GromNaN]

Fix PHPLIB-1128

generate_index_name() was originally made a utility function in bc65629 because it was shared by both the CreateIndexes (via IndexInput) and Count operations. It was later removed from Count in PHPLIB-285.

[jmikola]

Please check the two Psalm errors in functions.php. I'm not sure why those were caught in the PR but not locally when you regenerated the baseline.

[jmikola]

I noted that the original tests for the function were more comprehensive (the four possible type casts are applied to each index specification), but I don't think there's any significant benefit to doing that. Testing a basic index spec with the non-array types seems sufficient.

[GromNaN]

I'm fighting with psalm. baseline is not the same for my local PHP 8.2 and the CI PHP 7.4

Error: src/functions.php:86:12: MixedInferredReturnType: Could not verify return type 'array<array-key, mixed>|object' for MongoDB\apply_type_map_to_document (see https://psalm.dev/047)
Error: src/functions.php:95:12: MixedReturnStatement: Could not infer a return type (see https://psalm.dev/138)
Error: Process completed with exit code 2.

I decided to update the PHP version used in the CI in order to be able to generate the baseline from a Mac M1. Otherwise I cannot install the mongodb extension on PHP 7.4, I haven't figured out how to use a polyfill either.

Edit: I managed to manually edit psalm-pipeline.xml to pass the job. New ticket to update PHP version in CI: PHPLIB-1158