figure out dependency pinning strategy

[Remi-Gau]

suggestion:

• another PR to reduce size of requirements.txt
• if still needed switch to poetry or something else

Originally posted by @Remi-Gau in #104 (comment)

• for apps we may want to pin if not all at least many dependencies (not just the direct ones)
• for libraries we may want to only pin our direct dependencies

As a first approach we can in any case trim down our requirements.txt to the minimum and then either keep it that way or freeze all our dependencies depending on what we decide.

[Remi-Gau]

Maybe this issue should be moved to the project level as it may apply to several repos

[Remi-Gau]

see also https://github.com/neurobagel/project/issues/73

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