fix: address shellcheck warnings in create-adaptation-pr.sh

[jessealama]

Fixes #202

Addresses all shellcheck warnings in scripts/create-adaptation-pr.sh:

• Quote variables to prevent globbing/word splitting (SC2086)
• Use $(...) instead of legacy backticks (SC2006)
• Add -r flag to read commands (SC2162)

[fmontesi]

LGTM but I'll wait for @kim-em as well, since this might be interesting in general for other repos, too.

[arademaker]

@kim-em , can we accept this PR? It looks good to me.

[kim-em]

Corresponding PRs for the same fixes in batteries and mathlib4:

• fix: address shellcheck warnings in create-adaptation-pr.sh leanprover-community/batteries#1624 (batteries)
• fix: address shellcheck warnings in create-adaptation-pr.sh leanprover-community/mathlib4#34286 (mathlib4)

[kim-em]

Minor suggestion: the quoting style could be more consistent. The PR uses bump/"$BUMPVERSION" but the existing code already uses "bump/$BUMPVERSION" in other places (e.g., line 94).

For consistency, consider using:

# Instead of:
git checkout bump/"$BUMPVERSION" -- lean-toolchain
# Use:
git checkout "bump/$BUMPVERSION" -- lean-toolchain

Both work correctly, but quoting the whole string is more idiomatic and matches the existing style in the script.

I've updated the corresponding batteries and mathlib4 PRs to use the consistent style:

• fix: address shellcheck warnings in create-adaptation-pr.sh leanprover-community/batteries#1624
• fix: address shellcheck warnings in create-adaptation-pr.sh leanprover-community/mathlib4#34286

[kim-em]

Looks good to me, thanks for looking into this.

[chenson2018]

(I merged main so CI would let me put this on the merge queue)