RFR: 7903727: Remove the reliance on String Templates feature

[Nizar Benalla]

This PR aims to replace the usage of string templates with `String::format`, since there will be no string template feature in JDK 23.
I tried to keep similar indentation and convert them in-place, to make reviewing the changes easier.

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Commit messages:
 - convert string template usage to String::format

Changes: https://git.openjdk.org/jextract/pull/244/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=244&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903727
  Stats: 370 lines in 9 files changed: 62 ins; 4 del; 304 mod
  Patch: https://git.openjdk.org/jextract/pull/244.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/244/head:pull/244

PR: https://git.openjdk.org/jextract/pull/244