Resolves Gradle deprecation and language version warnings
```gradle
Java compiler version 21 has deprecated support for compiling with source/target version 8.
```
The `java.toolchain.languageVersion` syntax is a superset that covers the previous syntax `android.compileOptions`
Removes a number of unused functions from the file, renames remaining
functions to match our builder script prefix patterns, removes unused
imports from various build scripts (e.g. Android).
Fixes: #14453
Clarifies the confusing builder.inc.sh / build-utils.sh distinction by
giving the scripts more appropriate names. Most build scripts should use
builder-full.inc.sh; some helper scripts can use builder-basic.inc.sh.
Documented in resources/build/README.md.
Renames:
* resources/build/builder.inc.sh to resources/build/builder-full.inc.sh
* resources/build/build-utils.sh to resources/build/builder-basic.inc.sh
Other changes:
* Moves Android-specific functions out of builder-basic.inc.sh and into
android/build.sh.
* Renames functions in builder-basic.inc.sh
More functions may be moved from builder-basic.inc.sh into utils.inc.sh
or other scripts in the future.
Fixes: #14065
Build-bot: build all
Test-bot: skip
Adds documentation for various functions, removes unused functions (a
couple of simple, very lightly used functions were unDRYed; these could
go back the other way across all shell scripts if necessary). TODO items
noted for follow-up refactoring.
Fixes: #14275
Relates-to: #14269
Build-bot: build all
Test-bot: skip
Note: there is a bit of potential confusion about the difference between
/resources/builder.inc.sh (the full implementation for builder scripts),
and /resources/build/builder.inc.sh (the source script that builder
scripts should always use).
This allows us to make assumptions that will always be true for builder
scripts that may not be true for other scripts, such as setting base
folder.
Fixes#11324.
* Always `cd "$THIS_SCRIPT_PATH"`
* Remove unnecessary `cd` from all build.sh
* Remove unnecessary `set -eu` from all build.sh (and `# set -x`)
* Replace old build-utils.sh incantation in a few build.sh scripts
Fixes up scripts (except under /linux) to use `#!/usr/bin/env bash`
instead of `#!/bin/bash` or `#!/bin/sh` so that we don't end up with
the ancient version of bash supplied with macOS.
This became urgent with this PR, because of bash-4.xisms in
build-utils.sh, for example on line 572:
```
if [[ -v _builder_params[$e] ]]; then
```
